A lot of kids can't afford the game so they won't get it until xmas maybe not even at all and cod4 is going to be on sale so many new gamers will play it.
Just like Dopey said, some kids aren't as fortunate as others. They might not get MW2 for a while so they will continue playing COD4. Now World at War is another story. The MP for it was never that great so I think they will take a bigger hit than COD4 will. Nazi Zombies will still get some playing time but there numbers will drop as well.