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 * 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 subfamily * Zeolite A-4, a sodium-type molecular sieve 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 ''Antiemetics and antinauseants'', a subgroup of the Anatomical Therapeutic Chemical Classification System * Lipoxin A4, a lipoxin * Androstenedione, 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, 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 (IATA code: A4), a defunct Argentine airline * Azimuth (airline) (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, an 1852 German steam locomotive model * LNER Class A4, a class of steam locomotives, including the famous ''Mallard'' * PRR A4, an American Pennsylvania Railroad locomotive classification * Togoshi Station (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, a luxury compact executive car manufactured by Audi, introduced in 1994 * Arrows A4, 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 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 (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), the use of the musical note A to reference musical pitch * Réti Opening (Encyclopaedia of Chess Openings code: A04) * A4, a climbing route grade * 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