W4

W4 may refer to:
 * Form W-4, an American tax form
 * W4, IATA code of Wizz Air Malta
 * Heart Nebula or Westerhout 4, an interstellar cloud in Cassiopeia
 * London W4, a postcode district in west London, England
 * W4 (nuclear warhead), a planned variant of the 1949–1953 Mark 4 nuclear bomb
 * W4, complete oxidation of metal, a degree of meteorite weathering
 * WLLZ (FM), a radio station in Detroit, USA, known as W4 from 1966 to 2000
 * WWWW-FM or W4 Country, a country music radio station in Ann Arbor, USA
 * W4, then IATA code for LC Perú, a Peruvian airline which operated until 2018
 * Warwick W-4 Hot Canary, a racing biplane
 * W4 tram, a class of electric trams built by the Melbourne & Metropolitan Tramways Board.