Anyone check out the MGS4 gameplay trailers on www.gametrailers.com ?
The graphics look good but, damn the enemy AI seams really weak and stiff.
The game tends to have pretty good difficulty levels. MGS2 started off with a few questions to determine which level you should play and if you should get to see the ship section or not. Quite interesting approach that was.
Anyway, as far as I remember if you set MGS at the highest difficulty level, the AI really was very, very tough. If they'd spot or hear you (and they'd spot you very soon) they'd radio in for backup almost immediately, and very big and thorough search parties typically would find you.
But, it is also very much a game. Depending on the difficulty level you'll know that some places to hide will work or not, they'll spot you from a certain fixed distance, etc.
Don't know how it will be for MGS4, but don't dismiss what you see too easily.