I haven't played COD4 yet. It's all multi-player. The games I've gotten in on are the "Elimination" and "One Man Army" games. The "elimination" ones are two teams of players going against each other to get a set goal of kills first. Like first team to 100 kills. The "one man army" is basically everyone for themselves and first one to X amount of kills wins. You get experience points and "GP" points to buy different weapons, armor, weapon modifications, ect.
I have fun with it. It's completely free so it's worth at least checking out and see what you think.
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“Every Marine is, first and foremost, a rifleman. All other conditions are secondary.”...General A.M. Gray (29th Commandant of the USMC)