The budget for MGS4 is staggering, and the $60 price point will dramatically weaken it's potential sales. It will be another Gears of War. Huge on hype, but sell less than 5 million units (Which is ok with a realistic budget).
Overall MGS4 will probably sell 3-4 million units, and later on break past 8 million. The reason I feel this will eventually sell so many units is this will be the best -exclusive- PS3 game on the console next to God of War 3. Another bonus is I'm sure Sony covered a lot of the expenses to make MGS4 and GoW3. I can bet a million dollars that the game won't sell more than 5 million units the first month. But I'm not rich so my loss.