The high explosive C4, a plastic explosive, or plastic bonded explosive (PBX), is made with a combination of binding material (the plastic), and the explosive material cyclotrimethylene-trinitramine (C3H6N6O6).
Cyclotrimethylene-trinitramine is commonly referred to as RDX, which can stand for either royal demolition explosive, or research development explosive.
C4 is a very stable explosive. In an episode of Mythbusters they proved that C4 can actually be set on fire and not explode; it requires a blasting cap in order to detonate.