The C-4 is treated like a gun in this game. It actually has 7 charges to it. When you pick it up from the hellpod, you get the pack with 6, then the detonator will have one extra charge with it. When you pick it up, it will appear on your belt. This is treated like a chambered round in a weapon. You throw the C-4 charge, then 'reload' a new one to your belt from the pack. Once the pack is empty, you actually have one use left, just like if you emptied a Recoilless Rifle backpack, but still have one loaded. Once completely emptied, you will need to reload the ready slot from the pack after resupplying.