I doubt that MGS4 wont be making its way onto PC any time soon... I say this because of the shear size of the game. It was meant to have filled up an entire Dual Layer Blu Ray Disc, which is like 50GB or so.
At this point in time, the standard for PC hardware configurations will contain drives that play Dual Layer DVD's & I believe that it will be a while before Blu-Ray Drives become standard in any PC Setup, so its just not financially viable for them to Port a PC Version since the market just isnt there yet.
Naturally, PC's could handle this game quite well but for the time being, games will be stuck on Dual Layer DVD's because thats the medium that most people use right now.
But Im not going to rule it out for the future entirely, maybe if they come up with some super pimping compression techniques they could fit it onto a couple of discs. Personally, I dont really see much of a need for it to come onto the PC though, I wouldnt really enjoy playing it on my PC very much.