C4 x2

C4 x2 is a first tier perk in Call of Duty 4: Modern Warfare that gives a player two packs of Composition-4. This perk is available at level 4, and can be combined with the perk Sonic Boom to increase the killzone radius. C4, when deployed, can be activated by tapping the use button twice. However, the farther the player is from the C4 the longer it will take for the C4 to detonate. When a player is killed, the C4 that he/she has deployed will be removed from the game once he/she respawns.

Multiplayer Tactics
C4 is commonly used before players unlock Claymore x2. It is most useful in any objective matches, though it can be used otherwise. In Domination, C4 can be used to booby trap flags. A notification will inform players that a team's flag is being taken, so a player can simply place C4 near the flag then detonate it by pressing the use button twice when they receive the notification. C4 can be extremely useful in Search and Destroy because it can be thrown underneath the targets when prone. On defence, you can place one under either objective and not have to worry about defending it as much, or you can place one under each and get a guaranteed kill from anywhere on the map if the bomb is planted at either. On offence, it is also good for defending the bomb after it is planted, as the player can throw it under the objective and then hide. They can either hide nearby in an unimportant area (such as underneath Target A in Backlot) and listen for the sound of the bomb being picked up, or they can hide far away while watching the bomb (a sniper, for example) and detonate C4 instead of shooting, which would have given their position away.

C4 doesn't have to be a defensive weapon. Aside from luring enemies into planted explosives, it can also be used as an impromptu grenade, able to be thrown into buildings through windows, even into higher floors. When used in this way, it can be detonated more quickly than a grenade (which has a set fuse and must still be cooked before throwing), and lacks the metallic "ping" sound and indicator of a grenade, which may alert enemies and cause them to run away before it goes off. The lob-like trajectory of the C4's toss can also make it easier to get over fences and low walls than grenades, which may land some distance away from the base and miss the enemy as a result. Usually the only way it can be seen is by the blinking red dot on the center of the C4.

One of the most disputed tactics with this weapon is in the game modes Sabotage and Search and Destroy, in which the target becomes the planting spot. It is simple:

1. A player is the last on his team, and only 1 or 2 players are left on the other team. The 'good' player has some C-4.

2. This player goes to his post, and finds the target.

3. The player will now look for the space between the two crates on the pallet. This is where the C-4 goes.

4. He hides and equips his primary weapon.

5. He doubles taps "use" when the enemy is planting the bomb, then finishes off the bomb planter and any other opposing forces.

The C-4 is hard to see, and is often missed. Since they don't usually find out where it is, they almost never learn how to counter this tactic effectively.

Another tactic that is very uncommon in Call of Duty 4's multiplayer is to knife out the windows of a car, then throw a pack of C4 into the interior. Next, run to an area where you can see the rigged car, preferably equipped with a sniper rifle, wait for an enemy to walk by and then detonate the explosives.