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
 
Transport
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.
 
See also
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