# A4

A4 commonly refers to:
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 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 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 A$_4$, 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