D4

D4, D.IV, d4 or variants may refer to:

Science and medicine

 * 22-Dihydroergocalciferol, vitamin D4
 * D4-isoprostane, a type of isoprostane
 * ATC code D04, Antipruritics, including antihistamines, anesthetics, etc., a subgroup of the Anatomical Therapeutic Chemical Classification System
 * D04, Carcinoma in situ of skin ICD-10 code
 * Dopamine receptor D4, a human gene
 * Octamethylcyclotetrasiloxane, called D4

Mathematics

 * Dihedral group of order 4, otherwise known as the Klein four-group
 * Dihedral group of order 8, the symmetry group of a regular 4-gon
 * D4 (root system)

Technology and computing

 * D4 (programming language), a programming language of the Dataphor system
 * D4 video connector, a type of analog video connector found on Japanese consumer electronics
 * D4 framing standard, a framing standard for traditional time-division multiplexing

Aircraft

 * Albatros D.IV, a World War I experimental German fighter aircraft
 * Auster D.4, a 1960 two-seat British light aircraft
 * Dewoitine D.4, a Dewoitine aircraft
 * Dunne D.4, a 1907 British experimental aircraft
 * Fokker D.IV, a World War I German fighter biplane
 * Phönix D.IV, a KuKLFT D-class designation aircraft
 * Schütte-Lanz D.IV, a 1917 German fighter prototype aircraft
 * Siemens-Schuckert D.IV, a 1918 German fighter aircraft

Rail

 * Bavarian D IV, an 1878 German tank locomotive model
 * Bavarian D IV (Ostbahn), an 1867 German steam locomotive model
 * D4, the tank engine variant of the Victorian Railways Dd class locomotive in Australia
 * GSWRI Class D4, a Great Southern and Western Railway Irish steam locomotive
 * LNER Class D4, a class of British steam locomotives
 * PRR D4, an 1873 American steam locomotive model
 * SP&S Class D-4, a Spokane, Portland and Seattle Railway locomotive

Road

 * D4 motorway (Czech Republic), a motorway in the Czech Republic
 * D4 motorway (Slovakia), a motorway in Slovakia

Engines

 * Mercedes D.IV, a World War I German 8-cylinder, liquid-cooled inline aircraft engine
 * Mercedes D.IVa, a 1917 German 6-cylinder, water-cooled, inline aircraft engine

Media and entertainment

 * The D4, a rock band from Auckland, New Zealand
 * D4 Princess, a 1999 Japanese manga series
 * D4: Dark Dreams Don't Die, a video game by Hidetaka Suehiro for the Xbox One and PC
 * D4 (film), by Darrin Dickerson
 * D4 - Get Up and Dance, an Indian dance fiction television series
 * Diablo IV, an action role-playing game by Blizzard Entertainment
 * Dillinger Four, a punk rock band from Minneapolis, Minnesota

Other uses

 * Nikon D4, a flagship DSLR from Nikon
 * Caterpillar D4 a small crawler type tractor built by Caterpillar Inc.
 * HMAS Voyager (D04), a 1949 Australian Navy destroyer
 * HMS D4, a 1911 British submarine
 * Motorway D4 (Slovakia), a motorway in southwestern Slovakia
 * Peugeot D4, a French van sold between 1955 and 1965
 * Dublin 4, a Dublin postcode
 * d4, a 4-sided die
 * d4, the second most popular opening move in chess; begins the Queen's Pawn Game
 * D4, IATA code for Alidaunia airline
 * D4, commonly used jargon by fans for Ddu-Du Ddu-Du, a song by K-Pop group Blackpink.