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 LFG Roland D.VI, an aircraft powered by the German Mercedes D.III engine 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 upcoming action role-playing game by Blizzard Entertainment Other uses Nikon D4, a flagship DSLR from Nikon Caterpillar D4 a small crawler type tractor built by Caterpillar Inc. , a 1949 Australian Navy destroyer , a 1911 British submarine