Get better weapon as quickly as possible
At the start you only get a handful selection of weak weaponry with iron sights that limit you visibility and aiming. Fortunately, the battlefield is littered with casualties that can be used to your advantages. Grab the best available weapon you can get as quickly as possible then head for cover.
Taking cover smartly
Death is brutal in Call of Duty 4, a single shot can give a premature death. Pick you cover smartly, make sure it a hard cover not just a wooden fence or tin roof. The best cover is when it protect all of your sides, enemy may come from your six o'clock.
Prone often
When cover is not available nearby , prone will give you steady aim and present smallest possible silhoutte, which makes you harder to hit. Run quickly, prone, run quickly prone. Even prone in grasses will significantly increase your chance of survival. Prone among dead bodies is also great for camouflage.
Team play
John Rambo is already dead very early in this game, because he ignores team play. You may quickly outgunned and outnumbered when wandering alone in this game. Always walk behind a group of team mates, never on the front. Always check your six, someone may pop-up from behind and spray your teams with hot lead shower.
Dear old knife
Knife is very short-ranged yet kills instantly. When the enemy is very close, rifle may become unwieldy, knife is the best weapon to dispatch an enemy real quick, when walk inside building, be ready to quickly use knife when someone bump into you.
Time your special abilities
Use your special abilities smartly, use it the right moment. How to know the best moment to use these abilities? Use it when your option is to die or use everything available at your disposal.