A4

A4, A04, A.IV, A.4 or A-4 may refer to:

• A4 paper, a paper size prevalent in many countries (210 × 297 mm), defined in the ISO 216 international standard
• A4, a type of stainless steel, as defined by ISO 3506, equivalent to SAE steel grade 316L
• A-004, the sixth and final abort test of the Apollo spacecraft
• A4 – In the Dungeons of the Slave Lords, a 1981 module for the Dungeons and Dragons role playing game
• A4 road, in several countries
• ATC code A04 Antiemetics and antinauseants, a subgroup of the Anatomical Therapeutic Chemical Classification System
• Biu-Mandara A.4 languages, an Afro-Asiatic family of languages mainly spoken in Cameroon and Nigeria
• British NVC community A4 (Hydrocharis morsus-ranae – Stratiotes aloides community), a British Isles plants community
• Combretastatin A-4, a stilbenoid chemical compound
• M16A4, a variant of the American M16 rifle
• Route A4 (WMATA), a bus route operated by the Washington Metropolitan Area Transit Authority
• Apple A4, microprocessor used in Apple's iPad, iPhone 4, 4th generation iPod Touch and 2nd generation Apple TV.
• AMD A4, microprocessor
• A4Tech, company that produces computer peripherals

Aeronautics

• "A-4 Helldiver", the civil version of the Curtiss Falcon an attack aircraft manufactured by Curtiss Aircraft Company
• A-4 Skyhawk, a 1954 attack aircraft manufactured by Douglas Aircraft Company
• Breda A.4, a 1926 Italian biplane trainer aircraft
• Focke-Wulf A 4, a World War II German Focke-Wulf aircraft

Locomotives

• Bavarian A IV, a 1852 German steam locomotive model
• LNER Class A4, a class of steam locomotives, including the famous Mallard
• PRR A4, an American PRR steam locomotive

Other vehicles

• A-4, USS Moccasin (SS-5), a Plunger-class submarine of the United States Navy, which was renamed A-4 on 17 November 1911
• Arrows A4, a 1982 British racing car
• Audi A4, a 1994 Audi luxury compact executive car
• HMNZS Kahu (A04), a 1978 Royal New Zealand Navy Moa class inshore patrol vessel
• HMS A4, an A-class submarine of the Royal Navy
• Prussian A 4, a 1913 German railbus

Other uses

• List of A4 roads
• See A (musical note) for A4, in the fourth octave, at 440 Hertz, also known as A-440
• A04, Encyclopaedia of Chess Openings code for Réti Opening
• A4, a Grade (climbing)
• A4, Southern Winds Airlines, the defunct Argentine airline, as its IATA code
• A4, the V-2 rocket, as the actual model number for the German weapon
• A4, the code for permission to use specific land or premises for pubs and bars in town and country planning in the UK
• A4, the designation for air force headquarters in the NATO continental staff system concerned with logistics
  • In the United Kingdom, the A4 Air Logistics branch of the UK Joint Force Air Component Headquarters
• A4, the Alternating group acting on four elements
• A 4, see List of Swedish regiments for A4, a Swedish designation (4th Artillery Regiment) that has been used by the following units:
  • Gotlands artilleriregemente (unknown–1892)
  • Norrlands artilleriregemente (1893–1997)
• Subfamily A4, a Rhodopsin-like receptors subfamily
• A4 Pod, a well-known family of killer whales living in British Columbia coastal waters