A4

A4 commonly refers to:


 * A4 paper, 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
 * A4, the alternating group 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

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 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