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C4command, control, communications, and computers (US DoD)
C4Composition 4 (explosive)
C4Channel 4 (UK Terrestrial Television)
C4Carrera 4 (Porsche 911 model)
C4Middle C (piano keyboard)
C4Combat Casualty Care Course
C4Schedule IV Controlled Substance (USA)
C4Chaos Computer Club Cologne (German computer club in Cologne member of the CCC family)
C4Center for Clouds, Chemistry and Climate
C4Fourth-Generation Corvette (Chevrolet)
C4Trident I Missile
C4Controlled Collapse Chip Connect
C4Controlled Collapsed Chip Connection
C4Call Centre Consulting Connection (Canada)
C4Central CONUS Control Center (GPS)
C4Care, Competence, Control and Communications (Teksystems project management methodology)