MGS4 here. It felt like a step backwards: #3 introduced a few niggles, but the stealth was the purest yet, with the camo and all. It meant that sometimes you'd be inching towards a guard, when you make a noise. He walks over, stares at the spot you're hiding in, your trousers suddenly become warm and moist, he shrugs and walks off. It was tense and cunning. The 60s setting meant that there was no technobabble, which held #2 back, because no matter how shocking a plot revelation is, if you need to explain in inner workings of a deep sea submarine in order for me to comprehend it, I'll be mighty tempted to walk off to get some popcorn.
#4, being near-future again, brought back incomprehensible computer language, introduced pointless characters with a strange tendency for awkward social graces like Sunny, brought back globally loathed Raiden & co., made a painstaking effort to tie up every loose end known to man, and concluded in probably the least likely way that didn't involve a massive orgy. The gameplay was slighty tweaked -argubly for the worse-, but the main problem is that when the cutscenes are generally dull, 3times as long as before, and utterly unlikely to be cared about, the sense of being drawn in is quite shattered.