In this guide, I will give you some good hints to staying alive and doing well in Call of Duty 4 online multiplayer.
Don't use sprint constantly: Sprint is good for covering long distances in a short amount of time and avoiding shots from an enemy that you cannot reach but try to limit your sprinting. If you sprint, it takes longer to bring up your gun and your are more oblivious to things happening around you.
Reload: It is a good habit to reload your gun after a fight but if you are in an unsafe place, wait to reload till you get somewhere safe. If you are a good shot, you will still have more than enough shots for another enemy if one appears (except with the skorpion, that pretty much needs to be reloaded after every kill).
Use your surroundings: You have a much greater chance of surviving if you get behind cover when you are in a fight. Be aware of cars around you, if you see one that is on fire, get away, it will blow up soon, you don't want to be around it when it does.
Don't camp: If you stay in one spot long enough, after a few kills, your enemies will seek you out and will kill you. Instead of staying in one spot and getting owned, keep moving, remember to use cover.
Pick your fights: Know the range of your weapon, don't try to fight an opponent from a long distance with an mp5, it has far too much recoil to be effective at that distance.
Stealth is key: If you run into multiple enemies who still don't know you are there, use your knife. Unless you have a silencer on your weapon, your knife is your best bet at keeping your cover.
Use tactics: If you know a sniper is covering an area or another sniper spot, don't go there, try to sneak around from behind or just avoid that spot altogether. Move in groups, a single person will not be able to effectively assault a position. Wait till a teammate has a UAV out if you are unsure of where the enemy team is located to use an air strike, if not you may waste an airstrike that could have gotten you a helicopter. Wait till you have 4 kills to use your UAV, this will lessen the chances of it running out before you get an airstrike.
I hope this helps. I look forward to adding more in the future.