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

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
• ', 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