C-4, or Composition 4, is a plastic explosive consisting of explosive chemicals coated with a malleable plastic binder that makes them less sensitive to heat and shock. It looks and feels like white clay or putty.
Soldiers use C-4 packs that are about 1 inch thick and 2 inches wide, and are wrapped in olive-drab cellophane.
It generally takes a blasting cap or detonator to set one off. Soldiers in the field have been known to break a piece off and light it to heat their C rations without explosive consequences.
But when a pack is detonated on the ground, it can leave a blast crater 300 feet wide.
Source: "C-4 Explosive: Tales of a War Far Away"