It looks like CoD 4 will be set in an alternate universe where a Stalin-like Russian dictator and a Middle Eastern Warlord are allied. Missions will take place throughout the world and across different time periods, and players will assume the role of two main protagonists or minor characters depending on the mission. Infinity Ward has also focused more on multiplayer than with previous games.
Graphically, Call of Duty 4 looks set to impress. According to Shacknews, the game's engine renders "highly detailed foliage complete with fluttering butterflies in lush areas, particle-based clouds, a subtle depth of field effect when peering through a zoomed sniper scope, and convincingly flowing water." CoD 4 is scheduled to come out for the PC some time this fall.