A4 most often 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 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 on four elements * A4, a Stainless steel#Types of stainless steel, 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, 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 * ''V-2 rocket, 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 * SPAD S.A, S.A-4 or 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 PRR locomotive classification, 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, a luxury compact executive car manufactured by Audi, introduced in 1994 * Arrows A4, a 1982 British racing car * Prussian A 4, a 1913 German List of Prussian locomotives and railbuses#Railbuses, railbus

Roads and routes

* List of A4 roads * A4, a List of Metrobus routes (Washington, D.C.), 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 Staff (military)#Continental Staff System, continental staff system concerned with logistics ** In the United Kingdom, the A4 Air Logistics branch of the Joint Force Air Component Headquarters * A 4, see List of Swedish regiments for A4, a Swedish designation (4th Artillery Regiment) that has been used by: ** Gotlands artilleriregemente (unknown–1892) ** Norrlands artilleriregemente (1893–1997) * Douglas A-4 Skyhawk, 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, 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 A (musical note), La or A in the fourth octave * A440 (pitch standard), 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 United Kingdom, town and country planning in the UK * A4 Pod, a family of killer whales living in British Columbia coastal waters * A4, a 1981 Scourge of the Slave Lords#A4 In the Dungeons of the Slave Lords, 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