C4 has been scritpted to be used in a variety of ways.

By equiping either a vehicle or an enemy's dead body with C4 it recieves the "rigged" tag.
To equip a vehicle or dead enemy body simple examine their inventory ("x" is default) and transpher c4 from your inventory to theirs.
Once a dead body or vehicle is "rigged" it can be remotely detonated with the laptop.

Alternatively, C4, when held in the hand recieves the "fuse_blow" tag. If you open your inventory ("i" is default) and drag the c4 in your hand out of your inventory (it will be highlighted in your inventory). It will fall on the ground at your feet. You will know it was tagged correctly by the visual yellow pointer designed to let you know it was successfully placed.

A good tactic is to drag C4 your not holding out of your inventory onto the ground spaced just far enough apart. Then place a "fuse blow" C4 and a "C4 fuse" within range to activate a chain reaction.

"fuse blow" C4 can be detonated with the "C4 fuse" available from the Shop Box.
Also can also be targeted and remotely detonated using the laptop.