A4 most often refers to: *

A4 paper ISO 216 is an 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, including A4, the most commonly available paper si ...

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

Science and mathematics

* British NVC community A4 (''Hydrocharis morsus-ranae - Stratiotes aloides'' community), one type of Aquatic communities in the British National Vegetation Classification system * Combretastatin A-4, a stilbenoid chemical compound * ''A''₄, the

alternating group In mathematics, an alternating group is the 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 by or Basic pr ...

on four elements * A4, a type of stainless steel, as defined by ISO 3506, equivalent to SAE steel grade 316L * Subfamily A4, a rhodopsin-like receptors subfamily

Medicine

* ATC code A04 ''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 dehy ...

, an androgen steroid hormone

Transportation

Aeronautics and astronautics

* "A-4 Helldiver", the civil version of the Curtiss Falcon 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 developed for the United States Navy and United States Marine Corps in the early 1950s. The delta-winged, single turbojet engined Skyhawk was designed a ...

, a 1954 attack aircraft manufactured by Douglas Aircraft Company *

Breda A.4 The Breda A.4 was a biplane Trainer (aircraft), trainer produced in Italy in the mid-1920s. It was of conventional configuration with a two-bay Stagger (aviation), unstaggered wing cellule and seating for the pilot and instructor in tandem open c ...

, a 1926 Italian biplane trainer aircraft * '' Aggregat-4'', the technical name for V-2 ballistic missile * A4, Southern Winds Airlines, the defunct Argentine airline, as its IATA code * A4, Azimuth (airline), as its current IATA code * A-004, the sixth and final abort test of the Apollo spacecraft * S.A-4 or 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 inc ...

, an 1852 German steam locomotive model *

LNER Class A4 The Class A4 is a class of streamlined 4-6-2 steam locomotive 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 instantly recognisable, and ...

, a class of steam locomotives, including the famous ''Mallard'' * PRR A4, an American Pennsylvania Railroad locomotive classification * Togoshi Station, number A-04 on the Toei Asakusa Line

Ships

* ''A-4'', , a ''Plunger''-class submarine of the United States Navy, which was renamed ''A-4'' on 17 November 1911 * , a 1978 Royal New Zealand Navy ''Moa''-class inshore patrol vessel * , an A-class submarine of the Royal Navy * , a Belgian naval patrol vessel of World War II

Other vehicles

Audi A4 The Audi A4 is a line of compact executive cars produced since 1994 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 platform. The fir ...

, 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 (also known as Formula 1 or F1) is the highest class of international racing for open-wheel single-seater formula racing cars sanctioned by the Fédération Internation ...

, a 1982 British racing car * Prussian A 4, a 1913 German railbus

Roads and routes

List of A4 roads This is a list of roads designated A4. A4 is the name of several roads: * A004 road (Argentina), a road connecting Buenos Aires-La Plata highway with the Juan María Gutiérrez circle * A4 motorway (Austria), a road connecting Vienna and Nickelsd ...

* A4, a Metrobus route in Washington, D.C.

Military

* A4, the model number of the World War II-era German V-2 rocket * 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 militar ...

concerned with logistics ** In the United Kingdom, the A4 Air Logistics branch of the

Joint Force Air Component Headquarters The Joint Force Air Component Headquarters (JFACHQ) is the United Kingdom's deployable air command and control unit. The JFACHQ is run by the Royal Air Force with representation from the other services. The JFACHQ has members from the operat ...

* A 4, see

List of Swedish regiments This is a list of Swedish regiments and other military units ( divisions, brigades, battalions, companies) that have existed since the 16th century. Most formations have changed names several times during their existence. Listed here are commonly ...

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

Gotlands artilleriregemente The Gotland Artillery Regiment ( sv, Gotlands artilleriregemente, 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 ...

(unknown–1892) **

Norrlands artilleriregemente The Norrland Artillery Regiment ( sv, Norrlands artilleriregemente), designation A 4, was a Swedish Army artillery regiment that traced its origins back to the 19th century. The regiment's soldiers were originally recruited from the provinces of ...

(1893–1997) *

, an American attack aircraft introduced in 1956 * M16A4, a variant of the American M16 rifle * Leopard 1A4, a variant of the German Leopard 1 main battle tank *

Leopard 2A4 The Leopard 2 is a 3rd generation main battle tank originally developed by Krauss-Maffei in the 1970s for the West German army. The tank first entered service in 1979 and succeeded the earlier Leopard 1 as the main battle tank of the West Ger ...

, a variant of the German Leopard 2 main battle tank

Computing

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

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 use of the musical note A to reference musical pitch * A04, Encyclopaedia of Chess Openings code for Réti Opening * A4, a grade (climbing) * 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