Call of Duty 4 I find has better game play and MORE action, I also heard from lot's of people it has a better storyline.
CoD4 Multiplayer is good - the way people want it, I heard many complaints around CoD5 Multi-Player.
You know why they are making CoD6 Modern Warfare 2? Because CoD4 must have been their biggest sell out! Why? Because its different, most FPS you buy at stores and WWI/WWII but this stands out on the shelf, even more than halo!
So after that long review, I would say CoD4 Modern Warfare
CoD 4 owns COD5 that's why they are making Call of Duty 6 Modern Warfare 2.
Cause it's the best. Modern is better you must be getting tired of WWII game's, I know that you honour soldiers in those games but there are so many of them, what about honouring the modern troops as well?
IMO Call of Duty 4 is much better. The only thing that is worth buying W@W for is Zombie Nazi mode which is amazing. SO it really depends on what style you like more Modern or WWII Survival