At the current rate it'll reach 4M easily. I would be surprised if it didn't exceed 4M by a clear amount. In fact, even 5M-6M isn't impossible. It is very succesful even now, and when the platinum version is released, the game is bound to get increased sales. Anyway, just 4M is terrible underestimating.
I don't know about Konami's expectations. I guess MGS4 will meet them but can't be sure.







