First of all let me say that I absolutly love Metal Gear 4. The game is succeding in tying up the stroy of 3 previous games as well as delivering some a few new and interesting gameplay segments. So I don't mean to undervalue metal gear when I say this...
However there is nothing in this game that xbox360 can't handle. Below are 2 of the main reasons people say this cant be ported to 360 (other than contracts Sony may have with Konami ofcourse)
1. Size
Everyone says that if it be ported, it would have to be on 7-8 DL DVD disks. While no one has yet confirmed if mgs4 is dual layer bluray or sigle layer (25 gigs or 50 gigs...4 xbox disks or 8 xbox disks)...lets take that it is infact a 50 GB bluray game...
One must remember that the majority of metal gears size are the cutscences...gameplay alone could easily fit on to 1 DVD and have room left over. Therefore, we must remember that new compression codecs such as x264 are completely embarrassing Blu-ray video and sound. x264 takes a full Blu-ray film in HD, and compresses it to around 2-3GB. With same quality almost (95%). Same could be applied to the games cutscens bringing the game down to 2 or 3 DL DVD's maximum
Also, the fact is that MGS4 does game installs from 2 to 4 minutes between EVER ACT! So you can just stare at the wall for 3 minutes, or you can get up and swap the disk 2-3 times while playing the whole game (if this came out to 360).
So size is not the issue...
2. Visuals
I have not seen ANYTHING visually that makes MGS4 imposible to port to 360. Once cutscenes are taken out of the picture(which look amazing but are not hard to port), there really isn't anything in mgs4 that makes this game stand out visually from other top level visual games on the 360.
None of the structures take damage (other than that Raven boss which was designed specifically with that in mind) other than window and glass explosions, lighting doesn't adjust dynamically (there is shade and then there is sun...sun never moves), the bodes dissapear from the ground and there isn't many characters on screen at the same time...etc. These are the thigns that require lots of processing power... so visually it should not be a problem.
Therefore the only reasons this game can't come to the 360 are:
A) Kojima doesn't want to do it
B) Sony paid for complete exclusive