A4
A4 commonly refers to:

A4 paper, a paper size defined by the ISO 216 standard, measuring 210 × 297 mmA4 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

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 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

See also
4A (disambiguation)
A4A (disambiguation)
AIV (disambiguation)