A4 commonly refers to: *

A4 paper ISO 216 is an International Organization for Standardization, international standard for paper sizes, used around the world except in North America and parts of Latin America. The standard defines the "A", "B" and "C" series of paper sizes, wh ...

, a paper size defined by the ISO 216 standard, measuring 210 × 297 mm A4 and variants may also refer to:

Science and technology

* British NVC community A4 (''Hydrocharis morsus-ranae - Stratiotes aloides'' community), one type of

Aquatic communities in the British National Vegetation Classification system This article gives an overview of the aquatic communities in the British National Vegetation Classification system. Introduction The aquatic communities of the NVC were described in Volume 4 of '' British Plant Communities'', first published in ...

* Combretastatin A-4, a stilbenoid chemical compound * ''A''₄, the

alternating group In mathematics, an alternating group is the Group (mathematics), group of even permutations of a finite set. The alternating group on a set of elements is called the alternating group of degree , or the alternating group on letters and denoted ...

on four elements * A4, a type of stainless steel * Subfamily A4, a

rhodopsin-like receptors Rhodopsin-like receptors are a protein family, family of proteins that comprise the largest group of G protein-coupled receptors. Scope G-protein-coupled receptors, GPCRs, constitute a vast protein family that encompasses a wide range of funct ...

subfamily * Zeolite A-4, a sodium-type

molecular sieve A molecular sieve is a material with pores of uniform size comparable to that of individual molecules, linking the interior of the solid to its exterior. These materials embody the molecular sieve effect, in which molecules larger than the pore ...

also known as 4A

Computing

* Apple A4, a microprocessor used in some Apple products * AMD A4, a microprocessor * Samsung Galaxy A04, a smartphone manufactured by Samsung Electronics

Medicine

ATC code A04 A04A Antiemetics and antinauseants A04AA Serotonin (5-HT3) antagonists :A04AA01 Ondansetron :A04AA02 Granisetron :A04AA03 Tropisetron :A04AA04 Dolasetron :A04AA05 Palonosetron :A04AA55 Palonosetron, combinations A04AD Other antiemetics :A04AD01 ...

''Antiemetics and antinauseants'', a subgroup of the Anatomical Therapeutic Chemical Classification System * Lipoxin A4, a lipoxin *

Androstenedione Androstenedione, or 4-androstenedione (abbreviated as A4 or Δ4-dione), also known as androst-4-ene-3,17-dione, is an endogenous weak androgen steroid hormone and intermediate in the biosynthesis of estrone and of testosterone from dehydroe ...

, an androgen steroid hormone

Transportation

Aeronautics and astronautics

* A-4 Helldiver, the civil version of the

Curtiss Falcon The Curtiss Falcon was a family of military biplane aircraft built by the American aircraft manufacturer Curtiss Aeroplane and Motor Company during the 1920s. Most saw service as part of the United States Army Air Corps as observation aircraft ...

an attack aircraft manufactured by Curtiss Aircraft Company *

Douglas A-4 Skyhawk The Douglas A-4 Skyhawk is a single-seat subsonic carrier-capable light attack aircraft designed and produced by the American aerospace manufacturer Douglas Aircraft Company, and later, McDonnell Douglas. It was originally designated A4D und ...

, a 1954 attack aircraft manufactured by Douglas Aircraft Company * Breda A.4, a 1926 Italian biplane trainer aircraft * '' Aggregat-4'', the technical name for V-2 ballistic missile *

Southern Winds Airlines Southern Winds Airlines (also known as ''SW Airlines'') was an Argentine commercial air carrier that operated from 1996 until 2005. Southern Winds was founded by Juan Maggio, who served as president of the company. History Initially a domestic ...

(IATA code: A4), a defunct Argentine airline *

Azimuth (airline) Azimuth (, branding itself as azimuth) is a Russian joint-stock company airline operating since 2017 based in Platov International Airport in Rostov-on-Don, the Capital city, capital of Rostov Oblast. It is currently banned from flying in EU airs ...

(IATA code: A4), a Russian airline * A-004, the sixth and final abort test of the Apollo spacecraft * SPAD S.A-4, 1915 French reconnaissance fighter built by SPAD

Railways

Bavarian A IV Bavarian A IV engines were German 2-2-2 steam locomotives with the Royal Bavarian State Railways (''Königlich Bayerische Staatsbahn''). The vehicles were developed for night journeys and operations on the North-South Railway. In order to incr ...

, an 1852 German steam locomotive model *

LNER Class A4 The LNER Class A4 is a class of streamlined 4-6-2 steam locomotive, steam locomotives designed by Nigel Gresley for the London and North Eastern Railway in 1935. Their streamlined design gave them high-speed capability as well as making them ins ...

, a class of steam locomotives, including the famous ''Mallard'' * PRR A4, an American Pennsylvania Railroad locomotive classification *

Togoshi Station is a subway station on the Toei Asakusa Line, operated by the Tokyo Metropolitan Bureau of Transportation. It is located in Shinagawa, Tokyo, Japan. Its number is A-04. Station layout Togoshi Station is composed of a singular island platform ...

(station number A-04), Toei Asakusa Line, Tokyo

Ships

* USS ''A-4'' (SS-5), a ''Plunger''-class submarine of the United States Navy * HMNZS ''Kahu'' (A04), a 1978 Royal New Zealand Navy ''Moa''-class inshore patrol vessel * HMS ''A4'', an A-class submarine of the Royal Navy * Belgian ship ''A4'', a Belgian naval patrol vessel of World War II

Other vehicles

Audi A4 The Audi A4 is a line of luxury compact executive cars produced from 1994 to 2025 by the German car manufacturer Audi, a subsidiary of the Volkswagen Group. The A4 has been built in five generations and is based on the Volkswagen Group B platfor ...

, a luxury compact executive car manufactured by Audi, introduced in 1994 *

Arrows A4 The Arrows A4 was the car which the Arrows Formula One Formula One (F1) is the highest class of worldwide racing for open-wheel single-seater formula Auto racing, racing cars sanctioned by the Fédération Internationale de l'Automobile ...

, a 1982 British racing car * Prussian A 4, a Prussian railbus

Roads and routes

* List of A4 roads * A4, a Metrobus route in Washington, D.C., US

Military

* A4, the designation for air force headquarters in the NATO

continental staff system A military staff or general staff (also referred to as army staff, navy staff, or air staff within the individual services) is a group of officers, enlisted, and civilian staff who serve the commander of a division or other large milita ...

concerned with logistics ** A4 Air Logistics, a branch of the Joint Force Air Component Headquarters in the UK * A 4, see

List of Swedish regiments This is a list of Sweden, Swedish regiments. Most formations have changed names several times during their existence. Listed here are commonly used names. Regiments were the highest organized organic units in the Swedish Army from the time of Gu ...

for A4, a Swedish designation (4th Artillery Regiment) that has been used by: *

Gotland Artillery Regiment The Gotland Artillery Regiment (, designation A 7) was a Swedish Army artillery regiment that was in active service between 1811 and 2000. The regiment was based in Visby as part of the Gotland Garrison. History The regiment origins from the Art ...

(designation: A 4), until 1892 * Norrland Artillery Regiment (designation: A 4), 1893–1997 * M16A4, a variant of the American M16 rifle * Leopard 1A4, a variant of the German Leopard 1 main battle tank * Leopard 2A4, a variant of the German Leopard 2 main battle tank

Other uses

* A4 (classification), an amputee sport classification * A4 or A₄, the musical note La or A in the fourth octave *

A440 (pitch standard) A440 (also known as Stuttgart pitch) is the musical pitch corresponding to an audio frequency of 440 Hz, which serves as a tuning standard for the musical note of A above middle C, or A4 in scientific pitch notation. It is standardized by the ...

, the use of the musical note A to reference musical pitch *

Réti Opening The Réti Opening is a hypermodern chess opening whose "traditional" or "classic method" begins with the moves: : 1. Nf3 d5 : 2. c4 White attacks Black's pawn from the , which may occasion 2...dxc4. White may couple this plan with a fian ...

(Encyclopaedia of Chess Openings code: A04) * A4, a climbing route

grade Grade most commonly refers to: * Grading in education, a measurement of a student's performance by educational assessment (e.g. A, pass, etc.) * A designation for students, classes and curricula indicating the number of the year a student has reach ...

* A4, the code for permission to use specific land or premises for pubs and bars in town and country planning in the UK * A4 pod, a family of killer whales living in British Columbia coastal waters * A4, a 1981 component for the ''Scourge of the Slave Lord'' adventure in the ''Dungeons and Dragons'' role playing game * Biu-Mandara A.4 languages, a family of languages spoken in Cameroon and Nigeria * Vlad A4, Belarusian YouTuber * A4 Revolution, the 2022 COVID-19 protests in China