How do you detonate C4?

Composition C-4 is an insensitive plastic explosive. Insensitive means that it must be subjected to a high energy shock wave (physical shock, not electrical) in order for it to detonate. That shock is provided by a high energy blasting cap. Ordinary levels of impact, such as being dropped, thrown, kicked, or even struck by a fired bullet will not cause C-4 to detonate under normal conditions.