Rouché's theorem
K4 or K-4 may mean:
- K-4 (Kansas highway), a highway in Kansas
- K4 (programming language), the fourth version of the k programming language created by Arthur Whitney
- Daewoo K4, a 40 mm highspeed Automatic Grenade Launcher by use in the South Korean Army
- GSR Class K4, a Great Southern Railways 2-6-0 Irish steam locomotive
- HMS K4, a 1915 British K class submarine
- LNER Class K4, a British class of 2-6-0 steam locomotives
- Pennsylvania Railroad Type K4, American 4-6-2 steam locomotives
- London Buses route K4, a Transport for London contracted bus route
- vjoon K4 Publishing System, a paperless editorial workflow system, used in tandem with Adobe InDesign and Adobe InCopy (used by newspapers, magazines, book and corporate publishers). It is distributed in the Americas by Managing Editor Inc.
- USS K-4 (SS-35), a 1914 United States Navy K-class submarine
- K4, a Slovenian discothèque
- K4, an abbreviation for a four-man Sprint Kayak
- Koalisyon ng Katapatan at Karanasan sa Kinabukasan, a Philippine Political Coalition
- Gasherbrum II mountain
- K4, a model of the iconic British red telephone box
- the normal modal logic K4
- Kalitta Air IATA code
- Dilaudid, slang term for the narcotic analgesic
- Pre-Kindergarten education
- , standard notation for the complete graph having 4 vertices
- Kawai K4, a synthesizer
- K-4, an Indian submarine launched ballistic missile.
K 4 is a Swedish regimental designation (4th Cavalry Regiment) that has been used by the following units:
- Smålands husarregemente (1816–1927)
- Norrlands dragonregemente (1928–)