I don't think COD4 will be released on the Wii, the graphics on the next one are far superior to the 3rd so the Wii wouldn't be able to handle it.
They may release a toned down version with not so good graphics and some of the parts of the game cut out but not likely.
I'm a game-play kinda guy. I dont really worry about graphics any more. If I wanted the amazin graphics I would have bought a PS3 or 360, but I wanted a new way of playing games, and the Wii filled the gap. I'm sure thats why most people bought the Wii, not because of graphics.