A type of plastique (plastic explosive) that is extremely difficult to detect using inexpensive ($100,000 and under) detection methods.
Its composition, chemically, is approximately 90% RDX and 10% plastic (almost always polyisobutylene, an addition polymer of isobutylene) and inert solids.
C-4 is considered quite dangerous due to its use of RDX, a highly explosive and virtually undetectable nitramine compound.
A nitramine is an organic nitrate compound with the basic structure of ammonia (one central nitrogen atom) with nitro groups attached.
Generally any organic compound with nitro groups attached is somewhat explosive, but RDX is reasonably stable in C-4, but extraordinarily explosive when an electrical charge is applied to detonate it.
Current detection methods act similarly to bomb-sniffing dogs, taking a sample of air and analyzing it for the presence of nitrogen (or in more advanced detectors, the presence of only nitro groups). These methods, due to RDX's amine structure, are relatively ineffective in finding RDX (or C-4.)
Only extremely expensive and delicate optical detection devices can find RDX with a reasonable degree of accuracy, but even these methods are not fool-proof.
C-4, as Zorin discussed, is sometimes used in Hollywood as well as in numerous military operations, espionage, and an alarming number of terrorist attacks. It is moldable and extremely portable, making it a particular favorite explosive of the terrorists to whom it is available.