Unlike the Pipe Bomb, the C4 Explosives do not detonate when an enemy enters their proximity, instead they must be detonated by pressing the Alternate Fire button. If you have planted multiple C4 Explosives, they will detonate in the order they were placed.
C4 Explosives can stick to almost any surface, including zeds.
The Demolitionist can carry one extra C4 Explosive for every 5 levels they gain, for a maximum of 7 explosives at level 25.
Though it's unlikely you'd ever have that many, it is possible to place up to 24 C4 Explosives at once. If you try to place a 25th one, it will automatically detonate the first one you placed.
The C4 Explosives are the only weapons that can be bought from the Trader but not upgraded (not counting the M99 AMR which is already Tier 5). They were upgradable during the Beta, but it was deemed too powerful.