The C4 Explosives is the tier 2 Demolitionist weapon which is obviously a explosive type weapon. On fire throws a C4 Explosive on the ground (or any surface, including Zeds) leaving it there until the alt-fire is triggered causing the explosive to explode, and if there are more that one explosive they are detonated in order or time i.e oldest C4 is detonated.
The C4 has a short life-span in the field, particularly on longer waves as the C4 only has a total ammo of 2, which is highly non-sustainable unless one can find ammo boxes easily.
The C4 is commonly used for making traps detonated when a large Zed or cluster of Zeds approach killing them of with the C4's high damage (second only to the RPG-7 in the Demolitionist's arsenal)
The second tactic for using the C4 is to plant them on strong Zeds and when they attack, the C4 is detonated stumbling and knocking down the Zed, leaving them open to attacks or leaving a window for the player to run away though this can be suicidal if the Zed is extremely close since the explosive can injure the player or outright kill them.