HOME

TheInfoList



OR:

4 (four) is a number, numeral and
digit Digit may refer to: Mathematics and science * Numerical digit, as used in mathematics or computer science ** Hindu-Arabic numerals, the most common modern representation of numerical digits * Digit (anatomy), the most distal part of a limb, such ...
. It is the natural number following 3 and preceding 5. It is the smallest semiprime and composite number, and is considered unlucky in many East Asian cultures.


In mathematics

Four is the smallest composite number, its proper divisors being and . Four is the sum and product of two with itself: 2 + 2 = 4 = 2 x 2, the only number b such that a + a = b = a x a, which also makes four the smallest squared prime number p^. In Knuth's up-arrow notation, , and so forth, for any number of up arrows. By consequence, four is the only square one more than a prime number, specifically three. The sum of the first four prime numbers two + three +
five 5 is a number, numeral, and glyph. 5, five or number 5 may also refer to: * AD 5, the fifth year of the AD era * 5 BC, the fifth year before the AD era Literature * ''5'' (visual novel), a 2008 visual novel by Ram * ''5'' (comics), an awa ...
+
seven 7 is a number, numeral, and glyph. 7 or seven may also refer to: * AD 7, the seventh year of the AD era * 7 BC, the seventh year before the AD era * The month of July Music Artists * Seven (Swiss singer) (born 1978), a Swiss recording artist ...
is the only sum of four consecutive prime numbers that yields an
odd Odd means unpaired, occasional, strange or unusual, or a person who is viewed as eccentric. Odd may also refer to: Acronym * ODD (Text Encoding Initiative) ("One Document Does it all"), an abstracted literate-programming format for describing X ...
prime number,
seventeen Seventeen or 17 may refer to: *17 (number), the natural number following 16 and preceding 18 * one of the years 17 BC, AD 17, 1917, 2017 Literature Magazines * ''Seventeen'' (American magazine), an American magazine * ''Seventeen'' (Japanese m ...
, which is the fourth
super-prime Super-prime numbers, also known as higher-order primes or prime-indexed primes (PIPs), are the subsequence of prime numbers that occupy prime-numbered positions within the sequence of all prime numbers. The subsequence begins :3, 5, 11, 17, 31, ...
. Four lies between the first proper pair of
twin primes A twin prime is a prime number that is either 2 less or 2 more than another prime number—for example, either member of the twin prime pair (41, 43). In other words, a twin prime is a prime that has a prime gap of two. Sometimes the term ''twin pr ...
, three and
five 5 is a number, numeral, and glyph. 5, five or number 5 may also refer to: * AD 5, the fifth year of the AD era * 5 BC, the fifth year before the AD era Literature * ''5'' (visual novel), a 2008 visual novel by Ram * ''5'' (comics), an awa ...
, which are the first two Fermat primes, like
seventeen Seventeen or 17 may refer to: *17 (number), the natural number following 16 and preceding 18 * one of the years 17 BC, AD 17, 1917, 2017 Literature Magazines * ''Seventeen'' (American magazine), an American magazine * ''Seventeen'' (Japanese m ...
, which is the third. On the other hand, the square of four 42, equivalently the fourth power of two 24, is sixteen; the only number that has a^ = b^ as a form of factorization. Holistically, there are four elementary arithmetic
operations Operation or Operations may refer to: Arts, entertainment and media * ''Operation'' (game), a battery-operated board game that challenges dexterity * Operation (music), a term used in musical set theory * ''Operations'' (magazine), Multi-Man ...
in mathematics:
addition Addition (usually signified by the Plus and minus signs#Plus sign, plus symbol ) is one of the four basic Operation (mathematics), operations of arithmetic, the other three being subtraction, multiplication and Division (mathematics), division. ...
(+),
subtraction Subtraction is an arithmetic operation that represents the operation of removing objects from a collection. Subtraction is signified by the minus sign, . For example, in the adjacent picture, there are peaches—meaning 5 peaches with 2 taken ...
(−),
multiplication Multiplication (often denoted by the cross symbol , by the mid-line dot operator , by juxtaposition, or, on computers, by an asterisk ) is one of the four elementary mathematical operations of arithmetic, with the other ones being additi ...
(×), and division (÷); and four basic number systems, the real numbers \mathbb, rational numbers \mathbb, integers \mathbb, and natural numbers \mathbb. Each natural number divisible by 4 is a difference of squares of two natural numbers, i.e. 4x = y^z^. A number is a multiple of 4 if its last two digits are a multiple of 4. For example, 1092 is a multiple of 4 because . Lagrange's four-square theorem states that every positive integer can be written as the sum of at most four square numbers. Three are not always sufficient; for instance cannot be written as the sum of three squares. There are four
all-Harshad number In mathematics, a harshad number (or Niven number) in a given number base is an integer that is divisible by the sum of its digits when written in that base. Harshad numbers in base are also known as -harshad (or -Niven) numbers. Harshad number ...
s: 1, 2, ''4'', and 6. 12, which is divisible by four thrice over, is a Harshad number in all bases except octal. A four-sided plane figure is a quadrilateral or quadrangle, sometimes also called a ''tetragon''. It can be further classified as a
rectangle In Euclidean plane geometry, a rectangle is a quadrilateral with four right angles. It can also be defined as: an equiangular quadrilateral, since equiangular means that all of its angles are equal (360°/4 = 90°); or a parallelogram containi ...
or ''oblong'', kite, rhombus, and square. Four is the highest degree general polynomial equation for which there is a
solution in radicals A solution in radicals or algebraic solution is a closed-form expression, and more specifically a closed-form algebraic expression, that is the solution of a polynomial equation, and relies only on addition, subtraction, multiplication, divi ...
. The four-color theorem states that a
planar graph In graph theory, a planar graph is a graph that can be embedded in the plane, i.e., it can be drawn on the plane in such a way that its edges intersect only at their endpoints. In other words, it can be drawn in such a way that no edges cross ...
(or, equivalently, a flat map of two-dimensional regions such as countries) can be colored using four colors, so that adjacent vertices (or regions) are always different colors. Three colors are not, in general, sufficient to guarantee this. The largest planar complete graph has four vertices. A solid figure with four faces as well as four vertices is a tetrahedron, which is the smallest possible number of faces and vertices a polyhedron can have. The regular tetrahedron, also called a 3-
simplex In geometry, a simplex (plural: simplexes or simplices) is a generalization of the notion of a triangle or tetrahedron to arbitrary dimensions. The simplex is so-named because it represents the simplest possible polytope in any given dimension. ...
, is the simplest Platonic solid. It has four regular triangles as faces that are themselves at dual positions with the vertices of another tetrahedron. Tetrahedra can be inscribed inside all other four Platonic solids, and tessellate space alongside the regular octahedron in the alternated cubic honeycomb.
Four-dimensional space A four-dimensional space (4D) is a mathematical extension of the concept of three-dimensional or 3D space. Three-dimensional space is the simplest possible abstraction of the observation that one only needs three numbers, called ''dimensions'', ...
is the highest-dimensional space featuring more than three regular convex figures: *Two-dimensional: infinitely many regular polygons. *Three-dimensional: five regular polyhedra; the five Platonic solids which are the tetrahedron,
cube In geometry, a cube is a three-dimensional solid object bounded by six square faces, facets or sides, with three meeting at each vertex. Viewed from a corner it is a hexagon and its net is usually depicted as a cross. The cube is the only r ...
, octahedron, dodecahedron, and
icosahedron In geometry, an icosahedron ( or ) is a polyhedron with 20 faces. The name comes and . The plural can be either "icosahedra" () or "icosahedrons". There are infinitely many non- similar shapes of icosahedra, some of them being more symmetrica ...
. *Four-dimensional: six regular polychora; the
5-cell In geometry, the 5-cell is the convex 4-polytope with Schläfli symbol . It is a 5-vertex four-dimensional object bounded by five tetrahedral cells. It is also known as a C5, pentachoron, pentatope, pentahedroid, or tetrahedral pyramid. It i ...
, 8-cell or tesseract,
16-cell In geometry, the 16-cell is the regular convex 4-polytope (four-dimensional analogue of a Platonic solid) with Schläfli symbol . It is one of the six regular convex 4-polytopes first described by the Swiss mathematician Ludwig Schläfli in the mi ...
,
24-cell In geometry, the 24-cell is the convex regular 4-polytope (four-dimensional analogue of a Platonic solid) with Schläfli symbol . It is also called C24, or the icositetrachoron, octaplex (short for "octahedral complex"), icosatetrahedroid, oct ...
,
120-cell In geometry, the 120-cell is the convex regular 4-polytope (four-dimensional analogue of a Platonic solid) with Schläfli symbol . It is also called a C120, dodecaplex (short for "dodecahedral complex"), hyperdodecahedron, polydodecahedron, heca ...
, and 600-cell. The
24-cell In geometry, the 24-cell is the convex regular 4-polytope (four-dimensional analogue of a Platonic solid) with Schläfli symbol . It is also called C24, or the icositetrachoron, octaplex (short for "octahedral complex"), icosatetrahedroid, oct ...
, made of regular octahedra, has no analogue in any other dimension; it is
self-dual In mathematics, a duality translates concepts, theorems or mathematical structures into other concepts, theorems or structures, in a Injective function, one-to-one fashion, often (but not always) by means of an Involution (mathematics), involutio ...
, with its
24-cell honeycomb In Four-dimensional space, four-dimensional Euclidean geometry, the 24-cell honeycomb, or icositetrachoric honeycomb is a regular polytope, regular space-filling tessellation (or honeycomb (geometry), honeycomb) of 4-dimensional Euclidean space by ...
dual to the
16-cell honeycomb In Four-dimensional space, four-dimensional Euclidean geometry, the 16-cell honeycomb is one of the three regular space-filling tessellations (or honeycomb (geometry), honeycombs), represented by Schläfli symbol , and constructed by a 4-dimensiona ...
. *Five-dimensional and every higher dimension: three regular convex n- polytopes, all within the infinite family of regular n-
simplex In geometry, a simplex (plural: simplexes or simplices) is a generalization of the notion of a triangle or tetrahedron to arbitrary dimensions. The simplex is so-named because it represents the simplest possible polytope in any given dimension. ...
es, n-
hypercube In geometry, a hypercube is an ''n''-dimensional analogue of a square () and a cube (). It is a closed, compact, convex figure whose 1- skeleton consists of groups of opposite parallel line segments aligned in each of the space's dimensions, ...
s, and n- orthoplexes. The fourth dimension is also the highest dimension where regular self-intersecting figures exist: *Two-dimensional: infinitaly many regular
star polygon In geometry, a star polygon is a type of non-convex polygon. Regular star polygons have been studied in depth; while star polygons in general appear not to have been formally defined, certain notable ones can arise through truncation operations ...
s. *Three-dimensional: ''four'' regular
star polyhedra In geometry, a star polyhedron is a polyhedron which has some repetitive quality of nonconvexity giving it a star-like visual quality. There are two general kinds of star polyhedron: *Polyhedra which self-intersect in a repetitive way. *Concave p ...
, the regular Kepler-Poinsot star polyhedra. *Four-dimensional: ten regular star polychora, the Schläfli–Hess star polychora. They contain
cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery w ...
of Kepler-Poinsot polyhedra alongside regular tetrahedra,
icosahedra In geometry, an icosahedron ( or ) is a polyhedron with 20 faces. The name comes and . The plural can be either "icosahedra" () or "icosahedrons". There are infinitely many non- similar shapes of icosahedra, some of them being more symmetrica ...
and dodecahedra. *Five-dimensional and every higher dimension: zero regular star-polytopes; uniform star polytopes in dimensions n > 4 are the most symmetric, which mainly originate from
stellation In geometry, stellation is the process of extending a polygon in two dimensions, polyhedron in three dimensions, or, in general, a polytope in ''n'' dimensions to form a new figure. Starting with an original figure, the process extends specific el ...
s of regular n-polytopes. Altogether, sixteen (or 16 = 42) regular convex and star polychora are generated from symmetries of ''four'' (4) Coxeter Weyl groups and point groups in the fourth dimension: the \mathrm A_ simplex, \mathrm B_ hypercube, \mathrm F_ icositetrachoric, and \mathrm H_ hexacosichoric groups; with the \mathrm D_ demihypercube group generating two alternative constructions. There are also sixty-four (or 64 = 43) four-dimensional Bravais lattices, ''and'' sixty-four
uniform polychora In geometry, a uniform 4-polytope (or uniform polychoron) is a 4-dimensional polytope which is vertex-transitive and whose cells are uniform polyhedra, and faces are regular polygons. There are 47 non-prismatic convex uniform 4-polytopes. There ...
in the fourth dimension based on the same \mathrm A_, \mathrm B_, \mathrm F_ and \mathrm H_
Coxeter groups In mathematics, a Coxeter group, named after Harold Scott MacDonald Coxeter, H. S. M. Coxeter, is an group (mathematics), abstract group that admits a group presentation, formal description in terms of Reflection (mathematics), reflections (or Kal ...
, and extending to prismatic groups of uniform polyhedra, including one special non-Wythoffian form, the grand antiprism. There are also two infinite families of duoprisms and antiprismatic prisms in the fourth dimension. Four-dimensional differential manifolds have some unique properties. There is only one differential structure on \mathbb^n except when n = 4, in which case there are uncountably many. The smallest non- cyclic group has four elements; it is the Klein four-group. ''A'' alternating groups are not simple for values n4. Further extensions of the real numbers under Hurwitz's theorem states that there are four normed division algebras: the real numbers \mathbb, the complex numbers \mathbb C, the
quaternion In mathematics, the quaternion number system extends the complex numbers. Quaternions were first described by the Irish mathematician William Rowan Hamilton in 1843 and applied to mechanics in three-dimensional space. Hamilton defined a quatern ...
s \mathbb H, and the octonions \mathbb O. Under Cayley–Dickson constructions, the sedenions \mathbb S constitute a further fourth extension over \mathbb. The real numbers are ordered, commutative and
associative In mathematics, the associative property is a property of some binary operations, which means that rearranging the parentheses in an expression will not change the result. In propositional logic, associativity is a valid rule of replacement f ...
algebras, as well as alternative algebras with
power-associativity In mathematics, specifically in abstract algebra, power associativity is a property of a binary operation that is a weak form of associativity. Definition An algebra over a field, algebra (or more generally a magma (algebra), magma) is said to be ...
. The complex numbers \mathbb C share all four multiplicative algebraic properties of the reals \mathbb, without being ordered. The quaternions loose a further commutative algebraic property, while holding associative, alternative, and power-associative properties. The octonions are alternative and power-associative, while the sedenions are only power-associative. The sedenions and all further ''extensions'' of these four normed division algebras are solely power-associative with non-trivial zero divisors, which makes them non-division algebras. \mathbb has a vector space of dimension 1, while \mathbb C, \mathbb H, \mathbb O and \mathbb S work in
algebraic number field In mathematics, an algebraic number field (or simply number field) is an extension field K of the field of rational numbers such that the field extension K / \mathbb has finite degree (and hence is an algebraic field extension). Thus K is a f ...
s of dimensions 2, 4, 8, and 16, respectively.


List of basic calculations


Evolution of the Hindu-Arabic digit

Brahmic numerals The Brahmi numerals are a numeral system attested from the 3rd century BCE (somewhat later in the case of most of the tens). They are a non positional decimal system. They are the direct graphic ancestors of the modern Hindu–Arabic numeral s ...
represented 1, 2, and 3 with as many lines. 4 was simplified by joining its four lines into a cross that looks like the modern plus sign. The Shunga would add a horizontal line on top of the digit, and the Kshatrapa and Pallava evolved the digit to a point where the speed of writing was a secondary concern. The Arabs' 4 still had the early concept of the cross, but for the sake of efficiency, was made in one stroke by connecting the "western" end to the "northern" end; the "eastern" end was finished off with a curve. The Europeans dropped the finishing curve and gradually made the digit less cursive, ending up with a digit very close to the original Brahmin cross. While the shape of the character for the digit 4 has an ascender in most modern typefaces, in typefaces with text figures the glyph usually has a descender, as, for example, in . On the seven-segment displays of pocket calculators and digital watches, as well as certain optical character recognition fonts, 4 is seen with an open top. Television stations that operate on channel 4 have occasionally made use of another variation of the "open 4", with the open portion being on the side, rather than the top. This version resembles the
Canadian Aboriginal syllabics Canadian syllabic writing, or simply syllabics, is a family of writing systems used in a number of Indigenous Canadian languages of the Algonquian, Inuit, and (formerly) Athabaskan language families. These languages had no formal writing s ...
letter ᔦ. The magnetic ink character recognition "CMC-7" font also uses this variety of "4".


In religion


Buddhism

*
Four Noble Truths In Buddhism, the Four Noble Truths (Sanskrit: ; pi, cattāri ariyasaccāni; "The four Arya satyas") are "the truths of the Noble Ones", the truths or realities for the "spiritually worthy ones". Four Noble Truths: BUDDHIST PHILOSOPHY Encycl ...
Samudaya In Buddhism, the Four Noble Truths (Sanskrit: ; pi, cattāri ariyasaccāni; "The four Arya satyas") are "the truths of the Noble Ones", the truths or realities for the "spiritually worthy ones".[aFour Noble Truths: BUDDHIST PHILOSOPHY Encycl ...
,
Nirodha In Buddhism, nirodha, "cessation," "extinction," or "suppression," refers to the cessation or renouncing of craving and desire. It is the third of the Four Noble Truths,_stating_that_suffering_(dukkha.html" ;"title="Four Noble Truths: BUDDHIST P ...
, Noble Eightfold Path">Magga The Buddhist path (''marga'') to liberation, also referred to as awakening, is described in a wide variety of ways. The classical one is the Noble Eightfold Path, which is only one of several summaries presented in the Sutta Pitaka. A number of o ...
*Four sights – observations which affected Prince Siddhartha deeply and made him realize the sufferings of all beings, and compelled him to begin his spiritual journey—an old age, old man, a illness, sick man, a death, dead man, and an
ascetic Asceticism (; from the el, ἄσκησις, áskesis, exercise', 'training) is a lifestyle characterized by abstinence from sensual pleasures, often for the purpose of pursuing spiritual goals. Ascetics may withdraw from the world for their p ...
* Four Great Elementsearth, water, fire, and wind *
Four Heavenly Kings The Four Heavenly Kings are four Buddhist gods, each of whom is believed to watch over one cardinal direction of the world. In Chinese mythology, they are known collectively as the "Fēng Tiáo Yǔ Shùn" () or "Sìdà Tiānwáng" (). In the a ...
*
Four Foundations of Mindfulness ''Satipatthana'' ( pi, Satipaṭṭhāna, italic=yes; sa, smṛtyupasthāna, italic=yes) is a central practice in the Buddha's teachings, meaning "the establishment of mindfulness" or "presence of mindfulness", or alternatively "foundations of ...
– contemplation of the body, contemplation of feelings, contemplation of mind, contemplation of mental objects *
Four Right Exertions The Four Right Exertions (also known as, Four Proper Exertions, Four Right Efforts, Four Great Efforts, Four Right Endeavors or Four Right Strivings) (Pali: '; Skt.: ' or ') are an integral part of the Buddhist path to Enlightenment (understan ...
* Four Bases of Power * Four jhānas * Four arūpajhānas *
Four Divine Abidings The ''brahmavihārās'' (sublime attitudes, lit. "abodes of brahma") are a series of four Buddhist virtues and the meditation practices made to cultivate them. They are also known as the four immeasurables ( Sanskrit: अप्रमाण, ''a ...
loving-kindness Loving-kindness may refer to: * an English translation of Chesed ( he, חֶסֶד, also Romanized: ) is a Hebrew word that means 'kindness or love between people', specifically of the devotional piety of people towards God as well as of love o ...
,
compassion Compassion motivates people to go out of their way to relieve the physical, mental or emotional pains of others and themselves. Compassion is often regarded as being sensitive to the emotional aspects of the suffering of others. When based on n ...
, sympathetic joy, and
equanimity Equanimity (Latin: ''æquanimitas'', having an even mind; ''aequus'' even; ''animus'' mind/soul) is a state of inner peace, psychological stability and composure which is undisturbed by experience of or exposure to emotions, pain, or other phenom ...
* Four stages of enlightenmentstream-enterer, once-returner, non-returner, and arahant * Four main pilgrimage sitesLumbini,
Bodh Gaya Bodh Gaya is a religious site and place of pilgrimage associated with the Mahabodhi Temple Complex in Gaya district in the Indian state of Bihar. It is famous as it is the place where Gautama Buddha is said to have attained Enlightenment ( pi, ...
, Sarnath, and
Kusinara Kushinagar ( Hindustani: or ; Pali: ; Sanskrit: ) is a town in the Kushinagar district in Uttar Pradesh, India. It is an important and popular Buddhist pilgrimage site, where Buddhists believe Gautama Buddha attained ''parinirvana''. Etymo ...


Judeo-Christian symbolism

*The Tetragrammaton is the four-letter name of God. *
Ezekiel Ezekiel (; he, יְחֶזְקֵאל ''Yəḥezqēʾl'' ; in the Septuagint written in grc-koi, Ἰεζεκιήλ ) is the central protagonist of the Book of Ezekiel in the Hebrew Bible. In Judaism, Christianity, and Islam, Ezekiel is acknow ...
has a vision of four
living creatures In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and fungi; ...
: a man, a lion, an ox, and an eagle. *The four Matriarchs (foremothers) of Judaism are
Sarah Sarah (born Sarai) is a biblical matriarch and prophetess, a major figure in Abrahamic religions. While different Abrahamic faiths portray her differently, Judaism, Christianity, and Islam all depict her character similarly, as that of a piou ...
,
Rebekah Rebecca, ; Syriac: , ) from the Hebrew (lit., 'connection'), from Semitic root , 'to tie, couple or join', 'to secure', or 'to snare') () appears in the Hebrew Bible as the wife of Isaac and the mother of Jacob and Esau. According to biblical ...
, Leah, and Rachel. *The Four Species ( lulav,
hadass Hadass (Hebrew: הדס, pl. ''hadassim'' - הדסים) is a branch of the myrtle tree that forms part of the lulav used on the Jewish holiday of Sukkot. Hadass is one of the Four species (''arba'ah minim''–ארבעת המינים). T ...
, aravah and etrog) are taken as one of the
mitzvot In its primary meaning, the Hebrew word (; he, מִצְוָה, ''mīṣvā'' , plural ''mīṣvōt'' ; "commandment") refers to a commandment commanded by God to be performed as a religious duty. Jewish law () in large part consists of discus ...
on the Jewish holiday of
Sukkot or ("Booths, Tabernacles") , observedby = Jews, Samaritans, a few Protestant denominations, Messianic Jews, Semitic Neopagans , type = Jewish, Samaritan , begins = 15th day of Tishrei , ends = 21st day of Tishre ...
. ( Judaism) *The Four Cups of Wine to drink on the Jewish holiday of Passover. ( Judaism) *The Four Questions to be asked on the Jewish holiday of Passover. ( Judaism) *The Four Sons to be dealt with on the Jewish holiday of Passover. ( Judaism) *The Four Expressions of Redemption to be said on the Jewish holiday of Passover. ( Judaism) *The four Gospels:
Matthew Matthew may refer to: * Matthew (given name) * Matthew (surname) * ''Matthew'' (ship), the replica of the ship sailed by John Cabot in 1497 * ''Matthew'' (album), a 2000 album by rapper Kool Keith * Matthew (elm cultivar), a cultivar of the Ch ...
, Mark,
Luke People *Luke (given name), a masculine given name (including a list of people and characters with the name) *Luke (surname) (including a list of people and characters with the name) *Luke the Evangelist, author of the Gospel of Luke. Also known as ...
, and John. ( Christianity) *The Four Horsemen of the Apocalypse ride in the Book of Revelation. ( Christianity) *The four holy cities of Judaism: Jerusalem, Hebron,
Safed Safed (known in Hebrew language, Hebrew as Tzfat; Sephardi Hebrew, Sephardic Hebrew & Modern Hebrew: צְפַת ''Tsfat'', Ashkenazi Hebrew pronunciation, Ashkenazi Hebrew: ''Tzfas'', Biblical Hebrew: ''Ṣǝp̄aṯ''; ar, صفد, ''Ṣafad''), i ...
, and Tiberius


Hinduism

*There are four Vedas: Rigveda, Samaveda, Yajurveda and Atharvaveda. *In
Puruṣārtha ''Purushartha'' (Sanskrit: पुरुषार्थ, International Alphabet of Sanskrit Transliteration, IAST: ) literally means "object(ive) of men".Dharma Dharma (; sa, धर्म, dharma, ; pi, dhamma, italic=yes) is a key concept with multiple meanings in Indian religions, such as Hinduism, Buddhism, Jainism, Sikhism and others. Although there is no direct single-word translation for '' ...
, Artha, Kāma, Moksha. *The four stages of life
Brahmacharya ''Brahmacharya'' (; sa, ब्रह्मचर्य ) is a concept within Indian religions that literally means to stay in conduct within one's own Self. In Yoga, Hinduism, Buddhism and Jainism it generally refers to a lifestyle charac ...
(student life), Grihastha (household life), Vanaprastha (retired life) and Sannyasa (renunciation). *The four primary castes or strata of society:
Brahmana The Brahmanas (; Sanskrit: , ''Brāhmaṇam'') are Vedic śruti works attached to the Samhitas (hymns and mantras) of the Rig, Sama, Yajur, and Atharva Vedas. They are a secondary layer or classification of Sanskrit texts embedded within ea ...
(priest/teacher), Kshatriya (warrior/politician), Vaishya (landowner/entrepreneur) and Shudra (servant/manual laborer). *The
swastika The swastika (卐 or 卍) is an ancient religious and cultural symbol, predominantly in various Eurasian, as well as some African and American cultures, now also widely recognized for its appropriation by the Nazi Party and by neo-Nazis. It ...
symbol is traditionally used in
Hindu Hindus (; ) are people who religiously adhere to Hinduism.Jeffery D. Long (2007), A Vision for Hinduism, IB Tauris, , pages 35–37 Historically, the term has also been used as a geographical, cultural, and later religious identifier for ...
religions as a sign of good luck and signifies good from all four directions. *The god Brahma has four faces. *There are four ''
yuga A ''yuga'', in Hinduism, is generally used to indicate an age of time. In the ''Rigveda'', a ''yuga'' refers to generations, a long period, a very brief period, or a yoke (joining of two things). In the ''Mahabharata'', the words ''yuga'' and ...
s'': ''
Satya ''Satya'' (Sanskrit: सत्य; IAST: ''satya)'' is a Sanskrit word loosely translated as truth, essence. A. A. Macdonell, ''Sanskrit English Dictionary'', Asian Educational Services, , pp. 330–331 It also refers to a virtue in Indian relig ...
'', '' Dvapara'', ''
Treta ''Treta Yuga'', in Hinduism, is the second and second best of the four ''yugas'' (world ages) in a ''Yuga Cycle'', preceded by '' Krita (Satya) Yuga'' and followed by ''Dvapara Yuga''. ''Treta Yuga'' lasts for 1,296,000 years (3,600 divine years) ...
'' and '' Kali''


Islam

* Eid al-Adha lasts for four days, from the 10th to the 14th of
Dhul Hijja Dhu al-Hijja ( ar, ذُو ٱلْحِجَّة, translit=Ḏū al-Ḥijja, ), also spelled Zu al-Hijja, is the twelfth and final month in the Islamic calendar. It is a very sacred month in the Islamic calendar, one in which the '' Ḥajj'' (Pilgri ...
. *The four holy cities of Islam: Mecca, Medina, Jerusalem and Damascus. *The four tombs in the Green Dome: Muhammad, Abu Bakr, Umar ibn Khattab and Isa ibn Maryam (Jesus). *There are four
Rashidun , image = تخطيط كلمة الخلفاء الراشدون.png , caption = Calligraphic representation of Rashidun Caliphs , birth_place = Mecca, Hejaz, Arabia present-day Saudi Arabia , known_for = Companions of t ...
or Rightly Guided Caliphs: Abu Bakr, Umar ibn al-Khattab, Uthman ibn Affan and
Ali ibn Abi Talib ʿAlī ibn Abī Ṭālib ( ar, عَلِيّ بْن أَبِي طَالِب; 600 – 661 CE) was the last of four Rightly Guided Caliphs to rule Islam (r. 656 – 661) immediately after the death of Muhammad, and he was the first Shia Imam. ...
. *The Four Arch Angels in Islam are: Jibraeel (Gabriel), Mikaeel (Michael), Izraeel (Azrael), and Israfil (Raphael) *There are four months in which war is not permitted: Muharram, Rajab, Dhu al-Qi'dah and
Dhu al-Hijjah Dhu al-Hijja ( ar, ذُو ٱلْحِجَّة, translit=Ḏū al-Ḥijja, ), also spelled Zu al-Hijja, is the twelfth and final month in the Islamic calendar. It is a very sacred month in the Islamic calendar, one in which the ''Hajj, Ḥajj'' (P ...
. *There are four
Sunni Sunni Islam () is the largest branch of Islam, followed by 85–90% of the world's Muslims. Its name comes from the word '' Sunnah'', referring to the tradition of Muhammad. The differences between Sunni and Shia Muslims arose from a disagr ...
schools of fiqh: Hanafi, Shafi`i, Maliki and Hanbali. *There are four major Sunni Imams:
Abū Ḥanīfa Nuʿmān ibn Thābit ibn Zūṭā ibn Marzubān ( ar, نعمان بن ثابت بن زوطا بن مرزبان; –767), commonly known by his '' kunya'' Abū Ḥanīfa ( ar, أبو حنيفة), or reverently as Imam Abū Ḥanīfa by Sunni Musl ...
, Muhammad ibn Idris ash-Shafi`i, Malik ibn Anas and Ahmad ibn Hanbal. *There are four books in Islam: Taurait, Zaboor, Injeel, Quran. *Waiting for four months is ordained for those who take an oath for abstention from their wives. *The waiting period of the woman whose husband dies is four months and ten days. *When Abraham said: "My Lord, show me how You give life to the dead," Allah said: "Why! Do you have no faith?" Abraham replied: "Yes, but in order that my heart be at rest." He said: "Then take four birds, and tame them to yourself, then put a part of them on every hill, and summon them; they will come to you flying. l-Baqara 2:260*The respite of four months was granted to give time to the mushriks in Surah At-Tawba so that they should consider their position carefully and decide whether to make preparation for war or to emigrate from the country or to accept Islam. *Those who accuse honorable women (of unchastity) but do not produce four witnesses, flog them with eighty lashes, and do not admit their testimony ever after. They are indeed transgressors. n-Noor 24:4


Taoism

* Four Symbols of
I Ching The ''I Ching'' or ''Yi Jing'' (, ), usually translated ''Book of Changes'' or ''Classic of Changes'', is an ancient Chinese divination text that is among the oldest of the Chinese classics. Originally a divination manual in the Western Zho ...


Other

*In a more general sense, numerous mythological and cosmogonical systems consider Four corners of the world as essentially corresponding to the four points of the compass. *Four is the sacred number of the
Zia Zia or ZIA (also spelled Ziya, Ḍiya , Dia or Diya) may refer to: People * Zia (name), including a list of people and fictional characters with the name ** A romanization of the Wu ( Shanghainese) pronunciation of the Chinese surname Xie (謝) ...
, an indigenous tribe located in the U.S. state of New Mexico. *The Chinese, the Koreans, and the Japanese are superstitious about the number four because it is a homonym for "death" in their languages. *In Slavic mythology, the god
Svetovid Svetovit, Sventovit, Svantovit is the god of abundance and war, and the chief god of the Slavic tribe of the Rani, and later of all the Polabian Slavs. His organized cult was located on the island of Rügen, at Cape Arkona, where his main temp ...
has four heads.


In politics

* Four Freedoms: four fundamental freedoms that Franklin D. Roosevelt declared ought to be enjoyed by everyone in the world: Freedom of Speech, Freedom of Religion, Freedom from Want, Freedom from Fear. *
Gang of Four The Gang of Four () was a Maoist political faction composed of four Chinese Communist Party (CCP) officials. They came to prominence during the Cultural Revolution (1966–1976) and were later charged with a series of treasonous crimes. The gang ...
: Popular name for four Chinese Communist Party leaders who rose to prominence during China's Cultural Revolution, but were ousted in 1976 following the death of Chairman Mao Zedong. Among the four was Mao's widow, Jiang Qing. Since then, many other political factions headed by four people have been called "Gangs of Four".


In computing

*Four bits (half a byte) are sometimes called a
nibble In computing, a nibble (occasionally nybble, nyble, or nybl to match the spelling of byte) is a four-bit aggregation, or half an octet. It is also known as half-byte or tetrade. In a networking or telecommunication context, the nibble is oft ...
.


In science

*A tetramer is an
oligomer In chemistry and biochemistry, an oligomer () is a molecule that consists of a few repeating units which could be derived, actually or conceptually, from smaller molecules, monomers.Quote: ''Oligomer molecule: A molecule of intermediate relativ ...
formed out of four sub-units.


In astronomy

*Four terrestrial (or rocky) planets in the Solar System:
Mercury Mercury commonly refers to: * Mercury (planet), the nearest planet to the Sun * Mercury (element), a metallic chemical element with the symbol Hg * Mercury (mythology), a Roman god Mercury or The Mercury may also refer to: Companies * Merc ...
, Venus, Earth, and Mars. *Four giant gas/ice planets in the Solar System: Jupiter,
Saturn Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; h ...
, Uranus, and
Neptune Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times ...
. *Four of Jupiter's moons (the
Galilean moons The Galilean moons (), or Galilean satellites, are the four largest moons of Jupiter: Io, Europa, Ganymede, and Callisto. They were first seen by Galileo Galilei in December 1609 or January 1610, and recognized by him as satellites of Jupiter ...
) are readily visible from Earth with a hobby telescope. * Messier object M4, a magnitude 7.5 globular cluster in the constellation Scorpius. *The Roman numeral IV stands for subgiant in the
Yerkes spectral classification scheme In astronomy, stellar classification is the classification of stars based on their stellar spectrum, spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a Prism (optics), prism or diffraction grati ...
.


In biology

*Four is the number of nucleobase types in DNA and
RNA Ribonucleic acid (RNA) is a polymeric molecule essential in various biological roles in coding, decoding, regulation and expression of genes. RNA and deoxyribonucleic acid ( DNA) are nucleic acids. Along with lipids, proteins, and carbohydra ...
adenine, guanine, cytosine, thymine ( uracil in
RNA Ribonucleic acid (RNA) is a polymeric molecule essential in various biological roles in coding, decoding, regulation and expression of genes. RNA and deoxyribonucleic acid ( DNA) are nucleic acids. Along with lipids, proteins, and carbohydra ...
). *Many chordates have four feet, legs or leglike appendages ( tetrapods). *The
mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...
ian heart consists of four chambers. *Many
mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...
s (
Carnivora Carnivora is a Clade, monophyletic order of Placentalia, placental mammals consisting of the most recent common ancestor of all felidae, cat-like and canidae, dog-like animals, and all descendants of that ancestor. Members of this group are f ...
,
Ungulata Ungulates ( ) are members of the diverse clade Ungulata which primarily consists of large mammals with hooves. These include odd-toed ungulates such as horses, rhinoceroses, and tapirs; and even-toed ungulates such as cattle, pigs, giraffes, ...
) use four fingers for movement. *All insects with wings except flies and some others have four wings. *Insects of the superorder
Endopterygota Endopterygota (from Ancient Greek ''endon'' 'inner' + ''pterón'' 'wing' + New Latin ''-ota'' 'having'), also known as Holometabola, is a superorder of insects within the infraclass Neoptera that go through distinctive larval, pupal, and adult s ...
, also known as Holometabola, such as butterflies, ants, bees, beetles, fleas, flies, moths, and wasps, undergo
holometabolism Holometabolism, also called complete metamorphosis, is a form of insect development which includes four life stages: egg, larva, pupa, and imago (or adult). Holometabolism is a synapomorphic trait of all insects in the superorder Endopterygot ...
—complete metamorphism in four stages—from (1) embryo (ovum, egg), to (2) larva (such as grub, caterpillar), then (3) pupa (such as the chrysalis), and finally (4) the imago. *In the common ABO blood group system, there are four blood types (A, B, O, AB). * Humans have four canines and four wisdom teeth. *The cow's stomach is divided in four digestive compartments: reticulum, rumen, omasum and abomasum.


In chemistry

*
Valency Valence or valency may refer to: Science * Valence (chemistry), a measure of an element's combining power with other atoms * Degree (graph theory), also called the valency of a vertex in graph theory * Valency (linguistics), aspect of verbs re ...
of carbon (that is basis of life on the Earth) is four. Also because of its tetrahedral crystal bond structure, diamond (one of the natural allotropes of carbon) is the hardest known naturally occurring material. It is also the valence of silicon, whose compounds form the majority of the mass of the Earth's crust. *The atomic number of beryllium *There are four basic states of matter: solid,
liquid A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a (nearly) constant volume independent of pressure. As such, it is one of the four fundamental states of matter (the others being solid, gas, a ...
, gas, and
plasma Plasma or plasm may refer to: Science * Plasma (physics), one of the four fundamental states of matter * Plasma (mineral), a green translucent silica mineral * Quark–gluon plasma, a state of matter in quantum chromodynamics Biology * Blood pla ...
.


In physics

* Special relativity and general relativity treat nature as four- dimensional: 3D regular space and one-dimensional time are treated together and called spacetime. Also, any event ''E'' has a light cone composed of four zones of possible communication and cause and effect (outside the light cone is strictly incommunicado). *There are four fundamental forces ( electromagnetism,
gravitation In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stron ...
, the weak nuclear force, and the strong nuclear force). *In
statistical mechanics In physics, statistical mechanics is a mathematical framework that applies statistical methods and probability theory to large assemblies of microscopic entities. It does not assume or postulate any natural laws, but explains the macroscopic be ...
, the
four functions inequality 4 (four) is a number, numeral and digit. It is the natural number following 3 and preceding 5. It is the smallest semiprime and composite number, and is considered unlucky in many East Asian cultures. In mathematics Four is the smallest c ...
is an inequality for four functions on a finite distributive lattice.


In logic and philosophy

*The symbolic meanings of the number four are linked to those of the cross and the square. "Almost from prehistoric times, the number four was employed to signify what was solid, what could be touched and felt. Its relationship to the cross (four points) made it an outstanding symbol of wholeness and universality, a symbol which drew all to itself". Where lines of latitude and longitude intersect, they divide the earth into four proportions. Throughout the world kings and chieftains have been called "lord of the four suns" or "lord of the four quarters of the earth", which is understood to refer to the extent of their powers both territorially and in terms of total control of their subjects' doings. *The
Square of Opposition In term logic (a branch of philosophical logic), the square of opposition is a diagram representing the relations between the four basic categorical propositions. The origin of the square can be traced back to Aristotle's tractate ''On Interpre ...
, in both its Aristotelian version and its Boolean version, consists of four forms: A ("All ''S'' is ''R''"), I ("Some ''S'' is ''R''"), E ("No ''S'' is ''R''"), and O ("Some ''S'' is not ''R''"). *In regard to whether two given propositions can have the same truth value, there are four separate logical possibilities: the propositions are ''subalterns'' (possibly both are true, and possibly both are false); ''subcontraries'' (both may be true, but not that both are false); ''contraries'' (both may be false, but not that both are true); or ''contradictories'' (it is not possible that both are true, and it is not possible that both are false). * Aristotle held that there are basically four causes in nature: the material, the
formal Formal, formality, informal or informality imply the complying with, or not complying with, some set of requirements (forms, in Ancient Greek). They may refer to: Dress code and events * Formal wear, attire for formal events * Semi-formal attire ...
, the efficient, and the final. *The
Stoics Stoicism is a school of Hellenistic philosophy founded by Zeno of Citium in Athens in the early 3rd century BCE. It is a philosophy of personal virtue ethics informed by its system of logic and its views on the natural world, asserting that th ...
held with four basic categories, all viewed as bodies (substantial and insubstantial): (1) ''substance'' in the sense of substrate, primary formless matter; (2) ''quality'', matter's organization to differentiate and individualize something, and coming down to a physical ingredient such as ''pneuma'', breath; (3) ''somehow holding'' (or ''disposed''), as in a posture, state, shape, size, action, and (4) ''somehow holding'' (or ''disposed'') ''toward something,'' as in relative location, familial relation, and so forth. * Immanuel Kant expounded a table of judgments involving four three-way alternatives, in regard to (1) Quantity, (2) Quality, (3) Relation, (4) Modality, and, based thereupon, a table of four categories, named by the terms just listed, and each with three subcategories. *
Arthur Schopenhauer Arthur Schopenhauer ( , ; 22 February 1788 – 21 September 1860) was a German philosopher. He is best known for his 1818 work ''The World as Will and Representation'' (expanded in 1844), which characterizes the phenomenal world as the prod ...
's doctoral thesis was '' On the Fourfold Root of the Principle of Sufficient Reason''. * Franz Brentano held that any major philosophical period has four phases: (1) Creative and rapidly progressing with scientific interest and results; then declining through the remaining phases, (2) practical, (3) increasingly skeptical, and (4) literary, mystical, and scientifically worthless—until philosophy is renewed through a new period's first phase. (See Brentano's essay "The Four Phases of Philosophy and Its Current State" 1895, tr. by Mezei and Smith 1998.) *
C. S. Peirce Charles Sanders Peirce ( ; September 10, 1839 – April 19, 1914) was an American philosopher, logician, mathematician and scientist who is sometimes known as "the father of pragmatism". Educated as a chemist and employed as a scientist for t ...
, usually a trichotomist, discussed four methods for overcoming troublesome uncertainties and achieving secure beliefs: (1) the method of tenacity (policy of sticking to initial belief), (2) the method of authority, (3) the method of congruity (following a fashionable
paradigm In science and philosophy, a paradigm () is a distinct set of concepts or thought patterns, including theories, research methods, postulates, and standards for what constitute legitimate contributions to a field. Etymology ''Paradigm'' comes f ...
), and (4) the fallibilistic, self-correcting method of science (see " The Fixation of Belief", 1877); and four barriers to inquiry, barriers refused by the fallibilist: (1) assertion of absolute certainty; (2) maintaining that something is unknowable; (3) maintaining that something is inexplicable because absolutely basic or ultimate; (4) holding that perfect exactitude is possible, especially such as to quite preclude unusual and anomalous phenomena (see
F.R.L.
irst Rule of Logic An infrared search and track (IRST) system (sometimes known as infrared sighting and tracking) is a method for detecting and tracking objects which give off infrared radiation, such as the infrared signatures of jet aircraft and helicopters. IR ...
1899). * Paul Weiss built a system involving four modes of being: Actualities (substances in the sense of substantial, spatiotemporally finite beings), Ideality or Possibility (pure normative form), Existence (the dynamic field), and God (unity). (See Weiss's ''Modes of Being'', 1958). *
Karl Popper Sir Karl Raimund Popper (28 July 1902 – 17 September 1994) was an Austrian-British philosopher, academic and social commentator. One of the 20th century's most influential philosophers of science, Popper is known for his rejection of the cl ...
outlined a tetradic schema to describe the growth of theories and, via generalization, also the emergence of new behaviors and living organisms: (1) problem, (2) tentative theory, (3) (attempted) error-elimination (especially by way of critical discussion), and (4) new problem(s). (See Popper's ''Objective Knowledge'', 1972, revised 1979.) * John Boyd (military strategist) made his key concept the decision cycle or OODA loop, consisting of four stages: (1) observation (data intake through the senses), (2) orientation (analysis and synthesis of data), (3) decision, and (4) action. Boyd held that his decision cycle has philosophical generality, though for strategists the point remains that, through swift decisions, one can disrupt an opponent's decision cycle. *
Richard McKeon Richard McKeon (; April 26, 1900 – March 31, 1985) was an American philosopher and longtime professor at the University of Chicago. His ideas formed the basis for the UN's Universal Declaration of Human Rights. Life, times, and influences McKeo ...
outlined four classes (each with four subclasses) of modes of philosophical inquiry: (1) Modes of Being (Being); (2) Modes of Thought (That which is); (3) Modes of Fact (Existence); (4) Modes of Simplicity (Experience)—and, corresponding to them, four classes (each with four subclasses) of philosophical semantics: Principles, Methods, Interpretations, and Selections. (See McKeon's "Philosophic Semantics and Philosophic Inquiry" in ''Freedom and History and Other Essays'', 1989.) *
Jonathan Lowe Edward Jonathan Lowe (; 24 March 1950 – 5 January 2014), usually cited as E. J. Lowe but known personally as Jonathan Lowe, was a British philosopher and academic. He was Professor of Philosophy at Durham University. Biography Lowe was bor ...
(E.J. Lowe) argues in ''The Four-Category Ontology'', 2006, for four categories: ''kinds'' (substantial universals), ''attributes'' (relational universals and property-universals), ''objects'' (substantial particulars), and ''modes'' (relational particulars and property-particulars, also known as "
tropes Trope or tropes may refer to: Arts, entertainment, and media * Trope (cinema), a cinematic convention for conveying a concept * Trope (literature), a figure of speech or common literary device * Trope (music), any of a variety of different things ...
"). (See Lowe's "Recent Advances in Metaphysics," 2001
Eprint
*Four opposed camps of the morality and nature of evil:
moral absolutism Moral absolutism is an ethical view that some (potentially all) actions are intrinsically right or wrong. Stealing, for instance, might be considered to be always immoral, even if done for the well-being of others (e.g., stealing food to feed a s ...
,
amoralism Moral nihilism (also known as ethical nihilism) is the meta-ethical view that nothing is morally right or wrong. Moral nihilism is distinct from moral relativism, which allows for actions to be wrong relative to a particular culture or individ ...
,
moral relativism Moral relativism or ethical relativism (often reformulated as relativist ethics or relativist morality) is used to describe several philosophical positions concerned with the differences in moral judgments across different peoples and cultures. ...
, and
moral universalism Moral universalism (also called moral objectivism) is the meta-ethical position that some system of ethics, or a universal ethic, applies universally, that is, for "all similarly situated individuals", regardless of culture, race, sex, religio ...
.


In technology

*The resin identification code used in recycling to identify low-density polyethylene. *Most
furniture Furniture refers to movable objects intended to support various human activities such as seating (e.g., stools, chairs, and sofas), eating (tables), storing items, eating and/or working with an item, and sleeping (e.g., beds and hammocks). Fu ...
has four legs – tables, chairs, etc. *The
four color process Color printing or colour printing is the reproduction of an image or text in color (as opposed to simpler black and white or monochrome printing). Any natural scene or color photograph can be optically and physiologically dissected into three ...
( CMYK) is used for printing. *Wide use of
rectangle In Euclidean plane geometry, a rectangle is a quadrilateral with four right angles. It can also be defined as: an equiangular quadrilateral, since equiangular means that all of its angles are equal (360°/4 = 90°); or a parallelogram containi ...
s (with four angles and four sides) because they have effective form and capability for close adjacency to each other (houses, rooms, tables, bricks, sheets of paper, screens, film frames). *In the
Rich Text Format ) As an example, the following RTF code would be rendered as follows: This is some bold text. Character encoding A standard RTF file can only consist of 7-bit ASCII characters, but can use escape sequences to encode other characters. Th ...
specification, language code 4 is for the Chinese language. Codes for regional variants of Chinese are congruent to . * Credit card machines have four-twelve function keys. *On most phones, the 4 key is associated with the letters G, H, and I, but on the BlackBerry Pearl, it is the key for D and F. *On many computer keyboards, the "4" key may also be used to type the dollar sign ($) if the shift key is held down. *It is the number of bits in a
nibble In computing, a nibble (occasionally nybble, nyble, or nybl to match the spelling of byte) is a four-bit aggregation, or half an octet. It is also known as half-byte or tetrade. In a networking or telecommunication context, the nibble is oft ...
, equivalent to half a byte *In
internet slang Internet slang (also called Internet shorthand, cyber-slang, netspeak, digispeak or chatspeak) is a non-standard or unofficial form of language used by people on the Internet to communicate to one another. An example of Internet slang is "LOL" m ...
, "4" can replace the word "for" (as "four" and "for" are pronounced similarly). For example, typing "4u" instead of "for you". *In Leetspeak, "4" may be used to replace the letter "A". *The TCP/IP stack consists of four layers.


In transport

*Many internal combustion engines are called
four-stroke A four-stroke (also four-cycle) engine is an internal combustion (IC) engine in which the piston completes four separate strokes while turning the crankshaft. A stroke refers to the full travel of the piston along the cylinder, in either directio ...
engines because they complete one thermodynamic cycle in four distinct steps: Intake, compression, power, and exhaust. *Most vehicles, including
motor vehicle A motor vehicle, also known as motorized vehicle or automotive vehicle, is a self-propelled land vehicle, commonly wheeled, that does not operate on Track (rail transport), rails (such as trains or trams) and is used for the transportation of pe ...
s, and particularly cars/automobiles and light commercial vehicles have four road
wheel A wheel is a circular component that is intended to rotate on an axle Bearing (mechanical), bearing. The wheel is one of the key components of the wheel and axle which is one of the Simple machine, six simple machines. Wheels, in conjunction wi ...
s. *"
Quattro Quattro is Italian for the number four. Quattro may also refer to: People * "Quattro", a nickname of A. J. Foyt IV Fictional characters * Quattro Vageena or Quattro Bageena, an alias of Char Aznable in the ''Mobile Suit Gundam'' anime series ...
", meaning four in the Italian language, is used by Audi as a trademark to indicate that all-wheel drive (AWD) technologies are used on Audi-branded cars. The word "Quattro" was initially used by Audi in 1980 in its original 4WD coupé, the Audi Quattro. Audi also has a privately held subsidiary company called quattro GmbH. * List of highways numbered 4


In sports

* In the
Australian Football League The Australian Football League (AFL) is the only fully professional competition of Australian rules football. Through the AFL Commission, the AFL also serves as the sport's governing body and is responsible for controlling the laws of the gam ...
, the top level of
Australian rules football Australian football, also called Australian rules football or Aussie rules, or more simply football or footy, is a contact sport played between two teams of 18 players on an oval field, often a modified cricket ground. Points are scored by k ...
, each team is allowed 4 "
interchanges Interchange may refer to: Transport * Interchange (road), a collection of ramps, exits, and entrances between two or more highways * Interchange (freight rail), the transfer of freight cars between railroad companies * Interchange station, a rai ...
" (substitute players), who can be freely substituted at any time, subject to a limit on the total number of substitutions. *In baseball: **There are four bases in the game:
first base A first baseman, abbreviated 1B, is the player on a baseball or softball team who fields the area nearest first base, the first of four bases a baserunner must touch in succession to score a run. The first baseman is responsible for the majori ...
, second base, third base, and home plate; to score a run, an offensive player must complete, in the sequence shown, a circuit of those four bases. ** When a batter receives four pitches that the umpire declares to be " balls" in a single at-bat, a
base on balls A base on balls (BB), also known as a walk, occurs in baseball when a batter receives four pitches that the umpire calls '' balls'', and is in turn awarded first base without the possibility of being called out. The base on balls is defined in Se ...
, informally known as a "walk", is awarded, with the batter sent to first base. **For scoring, number 4 is assigned to the second baseman. **Four is the most runs that can be scored on any single at bat, whereby all three baserunners and the batter score (the most common being via a grand slam). **The fourth batter in the batting lineup is called the
cleanup hitter In baseball, a cleanup hitter is the fourth hitter in the batting order. The cleanup hitter is traditionally the team's most powerful hitter. His job is to "clean up the bases", i.e., drive in base runners. Theory The thinking behind the us ...
. *In basketball, the number four is used to designate the power forward position, often referred to as "the four spot" or "the four". *In
cricket Cricket is a bat-and-ball game played between two teams of eleven players on a field at the centre of which is a pitch with a wicket at each end, each comprising two bails balanced on three stumps. The batting side scores runs by striki ...
, a four is a specific type of scoring event, whereby the ball crosses the boundary after touching the ground at least one time, scoring four runs. Taking four wickets in four consecutive balls is typically referred to as a double hat trick (two consecutive, overlapping hat tricks). *In American Football teams get four downs to reach the line of gain. *In rowing, a four refers to a boat for four rowers, with or without coxswain. In rowing nomenclature, 4− represents a coxless four and 4+ represents a coxed four. *In rugby league: ** A try is worth 4 points. ** One of the two starting centres wears the jersey number 4. (An exception to this rule is the
Super League The Super League (officially known as the Betfred Super League due to sponsorship from Betfred and legally known as Super League Europe), is the top-level of the British rugby league system. At present the league consists of twelve teams, of wh ...
, which uses static squad numbering.) *In rugby union: ** One of the two starting locks wears the jersey number 4. ** In the standard bonus points system, a point is awarded in the league standings to a team that scores at least 4 tries in a match, regardless of the match result.


In other fields

* The phrase " four-letter word" is used to describe many swear words in the English language. *Four is the only number whose name in English has the same number of letters as its value. *Four (, formal writing: , pinyin sì) is considered an
unlucky number Numerology (also known as arithmancy) is the belief in an occult, divine or mystical relationship between a number and one or more coinciding events. It is also the study of the numerical value, via an alphanumeric system, of the letters in ...
in
Chinese Chinese can refer to: * Something related to China * Chinese people, people of Chinese nationality, citizenship, and/or ethnicity **''Zhonghua minzu'', the supra-ethnic concept of the Chinese nation ** List of ethnic groups in China, people of va ...
, Korean, Vietnamese and Japanese cultures mostly in Eastern Asia because it sounds like the word "death" (, pinyin sǐ). To avoid complaints from people with tetraphobia, many numbered product lines skip the "four": e.g. Nokia cell phones (there was no series beginning with a 4 until the
Nokia 4.2 The Nokia 4.2 is a Nokia-branded entry-level smartphone developed by HMD Global, running the Android operating system, which was released on 14 May 2019. Overview It features 5.71" 19:9 720x1520 pixels IPS LCD display, an octa-core Qualcomm Sn ...
), Palm PDAs, etc. Some buildings skip floor 4 or replace the number with the letter "F", particularly in heavily Asian areas. ''See tetraphobia'' and '' Numbers in Chinese culture''. *In Pythagorean
numerology Numerology (also known as arithmancy) is the belief in an occult, divine or mystical relationship between a number and one or more coinciding events. It is also the study of the numerical value, via an alphanumeric system, of the letters in ...
(a
pseudocience Pseudoscience consists of statements, beliefs, or practices that claim to be both scientific and factual but are incompatible with the scientific method. Pseudoscience is often characterized by contradictory, exaggerated or unfalsifiable claim ...
) the number 4 represents security and stability. *The number of characters in a canonical four-character idiom. *In the NATO phonetic alphabet, the digit 4 is called "fower". *In astrology, Cancer is the 4th astrological sign of the Zodiac. *In Tarot,
The Emperor ''The'' () is a grammatical article in English, denoting persons or things that are already or about to be mentioned, under discussion, implied or otherwise presumed familiar to listeners, readers, or speakers. It is the definite article in E ...
is the fourth trump or
Major Arcana The Major Arcana are the named or numbered cards in a cartomantic tarot pack, the name being originally given by occultists to the trump cards of a normal tarot pack used for playing card games. There are usually 22 such cards in a standard 78-car ...
card. *In '' Tetris'', a game named for the Greek word for 4, every shape in the game is formed of 4 blocks each. *4 represents the number of Justices on the
Supreme Court of the United States The Supreme Court of the United States (SCOTUS) is the highest court in the federal judiciary of the United States. It has ultimate appellate jurisdiction over all U.S. federal court cases, and over state court cases that involve a point o ...
necessary to grant a writ of
certiorari In law, ''certiorari'' is a court process to seek judicial review of a decision of a lower court or government agency. ''Certiorari'' comes from the name of an English prerogative writ, issued by a superior court to direct that the record of ...
(i.e., agree to hear a case; it is one less than the number necessary to render a majority decision) at the court's current size. *Number Four is a character in the book series '' Lorien Legacies.'' *In the
performing arts The performing arts are arts such as music, dance, and drama which are performed for an audience. They are different from the visual arts, which are the use of paint, canvas or various materials to create physical or static art objects. Perform ...
, the fourth wall is an imaginary barrier which separates the audience from the performers, and is "broken" when performers communicate directly to the audience.


In music

*In written music, common time is constructed of four beats per measure and a quarter note receives one beat. *In popular or modern music, the most common time signature is also founded on four beats, i.e., 4/4 having four quarter note beats. *The common major scale is built on two sets of four notes (e.g., CDEF, GABC), where the first and last notes create an
octave In music, an octave ( la, octavus: eighth) or perfect octave (sometimes called the diapason) is the interval between one musical pitch and another with double its frequency. The octave relationship is a natural phenomenon that has been refer ...
interval (a pair-of-four relationship). *The interval of a perfect fourth is a foundational element of many genres of music, represented in
music theory Music theory is the study of the practices and possibilities of music. ''The Oxford Companion to Music'' describes three interrelated uses of the term "music theory". The first is the "rudiments", that are needed to understand music notation (ke ...
as the tonic and
subdominant In music, the subdominant is the fourth tonal degree () of the diatonic scale. It is so called because it is the same distance ''below'' the tonic as the dominant is ''above'' the tonicin other words, the tonic is the dominant of the subdomina ...
relationship. Four is also embodied within the circle of fifths (also known as circle of fourths), which reveals the interval of four in more active harmonic contexts. *The typical number of movements in a
symphony A symphony is an extended musical composition in Western classical music, most often for orchestra. Although the term has had many meanings from its origins in the ancient Greek era, by the late 18th century the word had taken on the meaning com ...
. *The number of completed, numbered symphonies by
Johannes Brahms Johannes Brahms (; 7 May 1833 – 3 April 1897) was a German composer, pianist, and conductor of the mid- Romantic period. Born in Hamburg into a Lutheran family, he spent much of his professional life in Vienna. He is sometimes grouped wit ...
. *The number of strings on a violin, a viola, a cello, double bass, a cuatro, a typical bass guitar, and a ukulele, and the number of string pairs on a
mandolin A mandolin ( it, mandolino ; literally "small mandola") is a stringed musical instrument in the lute family and is generally plucked with a pick. It most commonly has four courses of doubled strings tuned in unison, thus giving a total of 8 ...
. *"Four calling birds" is the gift on the fourth day of Christmas in the carol "
The Twelve Days of Christmas The Twelve Days of Christmas, also known as Twelvetide, is a festive Christian season celebrating the Nativity of Jesus. In some Western ecclesiastical traditions, "Christmas Day" is considered the "First Day of Christmas" and the Twelve Days a ...
".


Groups of four

* Big Four (disambiguation) *Four basic operations of arithmetic:
addition Addition (usually signified by the Plus and minus signs#Plus sign, plus symbol ) is one of the four basic Operation (mathematics), operations of arithmetic, the other three being subtraction, multiplication and Division (mathematics), division. ...
,
subtraction Subtraction is an arithmetic operation that represents the operation of removing objects from a collection. Subtraction is signified by the minus sign, . For example, in the adjacent picture, there are peaches—meaning 5 peaches with 2 taken ...
,
multiplication Multiplication (often denoted by the cross symbol , by the mid-line dot operator , by juxtaposition, or, on computers, by an asterisk ) is one of the four elementary mathematical operations of arithmetic, with the other ones being additi ...
, division. * Greek classical elements ( fire, air, water, earth). * Four seasons:
spring Spring(s) may refer to: Common uses * Spring (season), a season of the year * Spring (device), a mechanical device that stores energy * Spring (hydrology), a natural source of water * Spring (mathematics), a geometric surface in the shape of a ...
,
summer Summer is the hottest of the four temperate seasons, occurring after spring and before autumn. At or centred on the summer solstice, the earliest sunrise and latest sunset occurs, daylight hours are longest and dark hours are shortest, wit ...
,
autumn Autumn, also known as fall in American English and Canadian English, is one of the four temperate seasons on Earth. Outside the tropics, autumn marks the transition from summer to winter, in September ( Northern Hemisphere) or March ( Sou ...
, winter. *
The Four Seasons (disambiguation) The Four Seasons, originally referring to the traditional seasons of spring, summer, autumn and winter (typical of a temperate climate), may refer to: Music * ''The Four Seasons'' (Vivaldi), a 1725 set of four violin concertos by Antonio Vival ...
*A leap year generally occurs every four years. *Approximately four weeks (4 times 7 days) to a lunar month ( synodic month = 29.53 days). Thus the number four is universally an integral part of primitive sacred calendars. *Four weeks of
Advent Advent is a Christian season of preparation for the Nativity of Christ at Christmas. It is the beginning of the liturgical year in Western Christianity. The name was adopted from Latin "coming; arrival", translating Greek ''parousia''. In ...
(and four Advent candles on the Advent wreath). *Four cardinal directions: north,
south South is one of the cardinal directions or Points of the compass, compass points. The direction is the opposite of north and is perpendicular to both east and west. Etymology The word ''south'' comes from Old English ''sūþ'', from earlier Pro ...
, east, west. *
Four Temperaments The four temperament theory is a proto-psychological theory which suggests that there are four fundamental personality types: sanguine, choleric, melancholic, and phlegmatic. Most formulations include the possibility of mixtures among the types w ...
: sanguine,
choleric The four temperament theory is a proto-psychological theory which suggests that there are four fundamental personality types: sanguine, choleric, melancholic, and phlegmatic. Most formulations include the possibility of mixtures among the types w ...
,
melancholic Melancholia or melancholy (from el, µέλαινα χολή ',Burton, Bk. I, p. 147 meaning black bile) is a concept found throughout History of medicine#Greece and Roman Empire, ancient, medieval medicine of Western Europe, medieval and Lear ...
,
phlegmatic The four temperament theory is a proto-psychological theory which suggests that there are four fundamental personality types: sanguine, choleric, melancholic, and phlegmatic. Most formulations include the possibility of mixtures among the types w ...
. *
Four Humors Humorism, the humoral theory, or humoralism, was a system of medicine detailing a supposed makeup and workings of the human body, adopted by Ancient Greek and Roman physicians and philosophers. Humorism began to fall out of favor in the 1850s ...
: blood, yellow bile, black bile, phlegm. *
Four Great Ancient Capitals of China This is a list of historical capitals of China. Four Great Ancient Capitals There are traditionally four major historical capitals of China referred to as the "Four Great Ancient Capitals of China" (). The four are Beijing, Nanjing, Luoyang and X ...
. *
Four-corner method The Four-Corner Method () is a character-input method used for encoding Chinese characters into either a computer or a manual typewriter, using four or five numerical digits per character. The Four-Corner Method is also known as the Four-Corner ...
. * Four Asian Tigers, referring to the economies of Hong Kong, Taiwan, South Korea, and Singapore * Cardinal principles. *
Four cardinal virtues The cardinal virtues are four virtues of mind and character in both classical philosophy and Christian theology. They are prudence, justice, fortitude, and temperance. They form a virtue theory of ethics. The term ''cardinal'' comes from the La ...
: justice, prudence, temperance, fortitude. *Four suits of playing cards: hearts, diamonds, clubs, spades. *Four nations of the United Kingdom: England, Wales, Scotland, Northern Ireland. *Four
provinces of Ireland There have been four Provinces of Ireland: Connacht (Connaught), Leinster, Munster, and Ulster. The Irish language, Irish word for this territorial division, , meaning "fifth part", suggests that there were once five, and at times Kingdom_of_ ...
:
Munster Munster ( gle, an Mhumhain or ) is one of the provinces of Ireland, in the south of Ireland. In early Ireland, the Kingdom of Munster was one of the kingdoms of Gaelic Ireland ruled by a "king of over-kings" ( ga, rí ruirech). Following the ...
, Ulster, Leinster, Connacht. *Four estates: politics, administration, judiciary, journalism. Especially in the expression " Fourth Estate", which means journalism. *
Four Corners The Four Corners is a region of the Southwestern United States consisting of the southwestern corner of Colorado, southeastern corner of Utah, northeastern corner of Arizona, and northwestern corner of New Mexico. The Four Corners area ...
is the only location in the United States where four states come together at a single point: Colorado, Utah, New Mexico, and Arizona. * Four Evangelists
Matthew Matthew may refer to: * Matthew (given name) * Matthew (surname) * ''Matthew'' (ship), the replica of the ship sailed by John Cabot in 1497 * ''Matthew'' (album), a 2000 album by rapper Kool Keith * Matthew (elm cultivar), a cultivar of the Ch ...
, Mark,
Luke People *Luke (given name), a masculine given name (including a list of people and characters with the name) *Luke (surname) (including a list of people and characters with the name) *Luke the Evangelist, author of the Gospel of Luke. Also known as ...
, and John * Four Doctors of Western Church – Saint Gregory the Great, Saint
Ambrose Ambrose of Milan ( la, Aurelius Ambrosius; ), venerated as Saint Ambrose, ; lmo, Sant Ambroeus . was a theologian and statesman who served as Bishop of Milan from 374 to 397. He expressed himself prominently as a public figure, fiercely promo ...
, Saint Augustine, and Saint Jerome * Four Doctors of Eastern Church – Saint John Chrysostom, Saint Basil the Great, and Gregory of Nazianzus and
Saint Athanasius Athanasius I of Alexandria, ; cop, ⲡⲓⲁⲅⲓⲟⲥ ⲁⲑⲁⲛⲁⲥⲓⲟⲩ ⲡⲓⲁⲡⲟⲥⲧⲟⲗⲓⲕⲟⲥ or Ⲡⲁⲡⲁ ⲁⲑⲁⲛⲁⲥⲓⲟⲩ ⲁ̅; (c. 296–298 – 2 May 373), also called Athanasius the Great, ...
* Four
Galilean moons The Galilean moons (), or Galilean satellites, are the four largest moons of Jupiter: Io, Europa, Ganymede, and Callisto. They were first seen by Galileo Galilei in December 1609 or January 1610, and recognized by him as satellites of Jupiter ...
of Jupiter – Io,
Europa Europa may refer to: Places * Europe * Europa (Roman province), a province within the Diocese of Thrace * Europa (Seville Metro), Seville, Spain; a station on the Seville Metro * Europa City, Paris, France; a planned development * Europa Cliff ...
, Ganymede, and Callisto * The
Gang of Four The Gang of Four () was a Maoist political faction composed of four Chinese Communist Party (CCP) officials. They came to prominence during the Cultural Revolution (1966–1976) and were later charged with a series of treasonous crimes. The gang ...
was a
Chinese communist The Chinese Communist Party (CCP), officially the Communist Party of China (CPC), is the founding and sole ruling party of the People's Republic of China (PRC). Under the leadership of Mao Zedong, the CCP emerged victorious in the Chinese Civil ...
political faction. * The
Fantastic Four The Fantastic Four is a superhero team appearing in American comic books published by Marvel Comics. The team debuted in ''The Fantastic Four'' #1 ( cover dated Nov. 1961), helping usher in a new level of realism in the medium. It was the first ...
: Mr. Fantastic, The Invisible Woman, The Human Torch, The Thing. * The Teenage Mutant Ninja Turtles: Leonardo, Michelangelo, Donatello, Raphael * The Beatles were also known as the "Fab Four": John Lennon, Paul McCartney, George Harrison, Ringo Starr. *
Gang of Four The Gang of Four () was a Maoist political faction composed of four Chinese Communist Party (CCP) officials. They came to prominence during the Cultural Revolution (1966–1976) and were later charged with a series of treasonous crimes. The gang ...
is a British post-punk
rock Rock most often refers to: * Rock (geology), a naturally occurring solid aggregate of minerals or mineraloids * Rock music, a genre of popular music Rock or Rocks may also refer to: Places United Kingdom * Rock, Caerphilly, a location in Wales ...
band formed in the late 1970s. * Four rivers in the
Garden of Eden In Abrahamic religions, the Garden of Eden ( he, גַּן־עֵדֶן, ) or Garden of God (, and גַן־אֱלֹהִים ''gan-Elohim''), also called the Terrestrial Paradise, is the Bible, biblical paradise described in Book of Genesis, Genes ...
(
Genesis Genesis may refer to: Bible * Book of Genesis, the first book of the biblical scriptures of both Judaism and Christianity, describing the creation of the Earth and of mankind * Genesis creation narrative, the first several chapters of the Book of ...
2:10–14):
Pishon The Pishon ( ''Pîšōn'') is one of four rivers (along with Hiddekel (Tigris), Perath (Euphrates) and Gihon) mentioned in the Biblical Book of Genesis. In that passage, a source river flows out of Eden to water the Garden of Eden and from ther ...
(perhaps the Jaxartes or
Syr Darya The Syr Darya (, ),, , ; rus, Сырдарья́, Syrdarjja, p=sɨrdɐˈrʲja; fa, سيردريا, Sirdaryâ; tg, Сирдарё, Sirdaryo; tr, Seyhun, Siri Derya; ar, سيحون, Seyḥūn; uz, Sirdaryo, script-Latn/. historically known ...
), Gihon (perhaps the Oxus or Amu Darya), Hiddekel ( Tigris), and P'rat (
Euphrates The Euphrates () is the longest and one of the most historically important rivers of Western Asia. Tigris–Euphrates river system, Together with the Tigris, it is one of the two defining rivers of Mesopotamia ( ''the land between the rivers'') ...
). * There are also four years in a single Olympiad (duration between the Olympic Games). Many major international sports competitions follow this cycle, among them the FIFA World Cup and its women's version, the FIBA World Championships for men and women, and the Rugby World Cup. * There are four limbs on the human body. * Four Houses of Hogwarts in the
Harry Potter ''Harry Potter'' is a series of seven fantasy literature, fantasy novels written by British author J. K. Rowling. The novels chronicle the lives of a young Magician (fantasy), wizard, Harry Potter (character), Harry Potter, and his friends ...
series: Gryffindor, Hufflepuff, Ravenclaw, Slytherin. * Four known continents of the world in the ''
A Song of Ice and Fire ''A Song of Ice and Fire'' is a series of epic fantasy novels by the American novelist and screenwriter George R. R. Martin. He began the first volume of the series, ''A Game of Thrones'', in 1991, and it was published in 1996. Martin, who init ...
'' series: Westeros, Essos, Sothoryos, Ulthos. * Each Grand Prix in
Nintendo is a Japanese Multinational corporation, multinational video game company headquartered in Kyoto, Japan. It develops video games and video game consoles. Nintendo was founded in 1889 as by craftsman Fusajiro Yamauchi and originally produce ...
's ''
Mario Kart is a series of racing games developed and published by Nintendo. Players compete in go-kart races while using various power-up items. It features characters and courses from the ''Mario'' series as well as other gaming franchises such as ''T ...
'' series is divided into four cups and each cup is divided into four courses. The Mushroom Cup, Flower Cup, Star Cup, and Special Cup make up the Nitro Grand Prix, while the Shell Cup, Banana Cup, Leaf Cup, and the Lightning Cup make up the Retro Grand Prix.


See also

* List of highways numbered 4


References

*Wells, D. '' The Penguin Dictionary of Curious and Interesting Numbers'' London: Penguin Group. (1987): 55–58


External links


Marijn.Org on Why is everything four?
by Penelope Merritt at samuel-beckett.net
The Number 4The Positive Integer 4
{{Authority control Integers