A type of of plastic explosive (soft and moldable explosive) of the Composition C line of US-specified explosives consisting primarily of RDX.
C4 is very stable and insensitive to most physical shocks. C4 cannot be detonated by a gunshot or by dropping it onto a hard surface. It does not explode when set on fire or exposed to microwave radiation. Detonation can only be initiated by a combination of extreme heat and a shockwave, such as when a detonator inserted into it is fired.
C4 usually appears as a wrapped block of white substance, sometimes with wiring or a timer rigged on it to trigger the explosion.
