The defining feature of a detonation is that the reaction is triggered by mechanical energy rather than thermal energy. So the reaction front progresses along with the shock wave. This is true of all detonations, not just C4. Gas mixtures (e.g. the fuel/air mix in your car engine’s combustion chambers, or an oxyacetylene mixture, or a hydrogen/oxygen mixture) can detonate as well. A deflagration is when the reaction front progresses on a thermal basis. Light a brick of C4 on fire? Deflagration. Whack it with a blasting cap? Detonation.
Adding more detonators ensures that at least one of them will start the show. Don’t expect a bigger bang, just more certainty that you’ll get a bang at all.
It’s a different way of measuring it. Detonation is the rapid breaking and dissaembly of the explosive’s molecules, which required more than just thermal energy (lighting something on fire). You need enough energy to forcibly break the first few molecules in the explosive, which will emit enough energy to break the next one, and so on. . . This sort of reaction is way faster than the speed of sound in the material (detonation of C4 is, IIRC, 28k-ish feet per second–sound is something way slower than that). Yes, there are chemical reactions going on while molecules are being ripped apart–oxidation of fuels. So, the chemical, thermal, and [some] mechanical reactions all combine to rip apart the next molecule down the chain. This is a gross, gross oversimplification for lo, I am not a chemist.
Deflagration, or just thermal breakdown, is too slow to emit the necessary energy to break the next molecule fast enough. It’s way, way slower than the speed of sound in the C4.
C4 is mostly RDX with some binders, plasticizers, taggants, and a little motor oil added into the mixture. I don’t have my hadnbook with me for exact mixture ratios, though. SEMTEX is the Eastern Bloc’s facsimile of C-4, but uses more PETN than RDX.