The C4 fuse can be purchased at the shop box. It will appear as a red medikit. Simply equip this item in your units hand and then open your inventory and drag it out. It will then become a fuse switch. the nearest placed "fuse blow" C4, if it is within 50 meters, will be highlighted with a yellow pointer and the switch will be highlighted with a white pointer. This indicated which c4 will be detonated on the next press of the fuse switch. The switch can be picked back up and taken with you.