There are a number of unlockables that you can obtain in multiplayer. For the most part, these are unlocked as you increase in level, meaning that no particularly special actions are required to unlock them; so long as you can kill a few dudes in every round, you'll eventually get the experience required to unlock them, even if you're otherwise a very bad player.
There are two types of challenges: weapon-specific and general training challenges.
Weapon challenges are based around the specific types of weapons that you unlock. Each weapon that you're capable of using in combat will have Marksman and Expert challenges for it; you can complete the Marksman challenges by killing enemies, while the Expert challenges can only be completed by getting headshots with the weapon. Marksman challenges will unlock new attachments for the given weapon, like reflex sights or silencers. Expert challenges will unlock new camo schemes for the weapon.
One unmentioned bonus is the ability to unlock new golden weapons. If you complete all of the Marksman and Expert challenges for a given class of weapons, you'll unlock a new golden weapon from that class. For instance, completing every single SMG Marksman and Expert challenge will unlock a new golden AK-47. These golden weapons don't offer you any new abilities or damage, but will definitely make you cut a dashing figure on the battlefield.
Training challenges force you to complete tasks on the battlefield in order to gain XP rewards. It's not necessary to complete training challenges in order to gain levels - killing fools is enough for that - but being aware of what you need to do to unlock challenges will boost the rate at which you gain XP.
Early on, the challenges are easy, with such tasks as blowing up cars or killing enemies with the knife. As you gain levels, though, more challenges are unlocked, and they get progressively crazier. At the later levels, you'll be faced with challenges like killing an enemy, picking up his or her weapon, then killing them with their own weapon after they respawn, or shooting an explosive through a wall to kill an enemy on the other side.
Training challenges are purely for the experience; they don't offer any unlockables that we know of.
We're not going to detail all of the weapon unlocks here, although it's worth pointing out that you can be very effective with even the basic weapons that are available at the first level. If you want a decent weapon that'll be effective on most maps, go for the MP5 submachine gun and get used to its firing mechanisms; it's pretty effective on the smaller maps and can take down enemies at range if you aim well. As you get more familiar with the weapon types, feel free to switch over to an assault rifle or light machine gun. Shotguns and sniper rifles will be situational weapons that are probably only going to be useful on certain maps, but can still be deadly in the hands of a proficient player.
You can equip up to three perks at a time in Call of Duty 4. These perks help you customize your player with different effects. A few perks are available at level one, but as you increase in level, more will become available to you.
Note that attaching a grenade launcher to an assault rifle is possible; if you do so, it will replace whatever perk you select here.
- C4 x2: You gain two pieces of C4. You can throw these with the left trigger and detonate them with the right trigger, but you'll be really exposed to enemy fire when you run around with them. Requires a bit of trickery to use well, but if you're good at getting enemies to chase you, then you can leave them special surprises.
- Special Grenades x3: You'll gain an extra three special grenades. Can choose from flash or stun grenades, but not smoke; smoke is only available if you don't use this perk.
- RPG-7 x2: Gives you an RPG and two rounds. Handy for shooting down enemy helicopters, but somewhat more difficult to use against actual players due to the time it takes to actually fire the weapon. If you want to use it on players (for the challenges), try using the RPG on Domination maps, where you're relatively sure to find clusters of foes around flags.
- Unlockable Claymore x2: Two plantable claymores. Useful for covering your back if you're going to camp up and snipe, or to defuse a thorny situation if an enemy is chasing you. Also handy to protect objectives in objective-based gameplay modes. Note that claymores take a second to pop into the ground, so it's tough to use them unless you're relatively clear of enemy influence. The red lines that denotes claymore positions will be a dead giveaway to your foes, if they're visible; try to hide claymores around a corner instead of putting them right in the middle of doorways. Also note that claymores despawn when you die, or shortly thereafter, so try to stay alive if you want to get kills with them!
- Unlockable Frag x3: More frag grenades for you. Handy if you're a throw-and-pray style.
- Unlockable Bandolier: Extra ammunition for primary and secondary weapons. Might be handy if you're an expert at surviving for long periods of time, but in most cases something like extra frag grenades will be more handy to you before you die, especially in the deathmatch modes.
- Unlockable Bomb Squad: You'll be able to see enemy C4 and Claymores on your minimap. Handy if you run into those explosives on a regular basis, but again this will depend on the gametype that you're playing. Domination and Headquarters will often see more use of these explosives than other gametypes will.
- Stopping Power: Increased bullet damage. A nice perk that'll be effective in almost any situation, although it's not like your enemies will require many bullets to kill normally.
- Juggernaut: Increase health. Again, pretty handy, but you'll still die pretty quickly if an enemy gets a bead on you.
- Sonic Boom: Higher damage from explosive weapons, including C4, frag grenades, RPG rounds, and claymores. This also increases the damage radius. Combine it with the Frag x3 Perk 1 for extra fun, especially in deathmatch or TDM.
- Unlockable Sleight of Hand: Helps you reload more quickly. This is a very, very useful perk, but only for certain weapons. SMGs and assault rifles stand the most to gain from it, since you'll be chewing through ammo very quickly and will have to reload more often. In the case of LMGs, they take a very long time to reload, but in most cases you won't be able to chew through a whole case of ammo before you get killed, at least in more frenetic gametypes. And if you wind up having to reload an LMG, you're likely to either get killed (in deathmatch) or will be safe enough to where the reloading time-saving from Sleight of Hand won't really help all that much.
- Unlockable Double Tap: Increases rate of fire. For SMGs this will simply chew through your ammo clip that much more quickly, but for slow-firing weapons, like some of the light machine guns, it can let you put a lot more metal in the air.
- Unlockable Overkill: Allows you to carry two primary weapons instead of a primary weapon and a pistol. Handy on maps where you think you may need to fire at close range and long range, or if you want to work on two types of weapon challenges at the same time, but in most cases you'll want to pick a weapon and stick with it.
- Unlockable UAV Jammer: Your character does not show up on enemy UAV sweeps. Pretty handy, since most big games will have more or less constant UAV sweeps going down.
- Extreme Conditioning: You can sprint for longer distances.
- Steady Aim: You're more accurate when firing from the hip. Handy if you want to run-and-gun, but you still shouldn't expect to get headshots from across the map unless you use your sights.
- Deep Impact: Bullets will penetrate thicker materials. Allows you to hit enemies on the other side of some walls. Depending on the level, this can be really handy, but it's difficult to know when it'll work without a lot of experience.
- Eavesdrop: Allows you to hear the voice chat of enemy soldiers as you get close to them.
- Unlockable Last Stand: When you take lethal damage, you'll drop to the ground, pull out your pistol, and gain a few seconds in which to try and hit your killer with your sidearm. This only does you any good if your enemy killed you up-close; if they hit you from a distance it won't be possible to hit them, and if they notice you rolling around with your pistol they can continue to fire on you and kill you for real. Also, most good players will know that you have this perk when it activates, since they won't get the experience from the kill until you completely die, so they'll either run and let you bleed out or finish you off with another burst.
- Unlockable Martyrdom: When you die, you drop a live grenade directly on your body. Probably handier than Last Stand, since your opponents will immediately get the experience gain and think you're dead (since you are), but the sound the grenade makes when it drops will tip off better players and encourage them to run. Still, there's a good chance that they won't notice the sound in the heat of the battlefield, or will have migrated from Halo and think they'll be cool by teabagging you, making this perk an amusing way to exact your revenge. You do get experience and kill credits for anyone the grenade kills, even though you'll be dead when it explodes.
- Unlockable Iron Lungs: You can hold your breath for a longer period time when sniping. Only handy for snipers, obviously, but pretty handy for them.
- Unlockable Dead Silence: You make less sound when you move. The last perk you'll unlock, and arguably one of the best if you like sneaking up on people, but keep in mind that you'll still show up on your opponent's minimaps
- Downloadable Game