MGS4 is a great game, shame it's not it's own game.
MGS4 is, mechanically, an improved, westernized version of Metal Gear Solid 3. They fixed up the previously horrible shooting controls, simplified the camouflage, and removed the survival elements. It's just a shame that they didn't think to transfer the greatest part of MGS3 - the amazing story.
Allow me to clarify:
Metal Gear Solid 4 has amazing cutscenes with amazing animation and amazing voice acting. What Metal Gear Solid 4 does not have is good writing. The plot is above average for a game, but the dialogue is pretty terrible most of the time, and you can tell it has been translated from Japanese. Maybe the dialogue sounds great in Japanese, but I don't speak Japanese. The reason MGS3's story was the best in the series was because it didn't rely on any Applied Phlebotinum (tvtropes.org, search for that if you don't know what it maens), and the ending was flawless.
Allow me to summarize the story of Metal Gear Solid 4:
Nanomachines nanomachines nanomachines nanomachines.
At any time during the game, if you're wondering about something, just keep in mind that every single plot point is explained away with a hand-wave and some technobabble about nanomachines.
I was about to summarize the ending too, but then I realised it would take 4000 words to do so. Because, much like LOTR: ROTK, the ending goes on. And on. And on. And on. Speaking of on and on, I don't need to tell most of you that the cutscenes drag on and on, forever and ever. This has been a hallmark of the series, and it's a love it or hate it thing. Personally, I loved it when it was good, but when it is bad (and sometimes it IS bad, especially when they use a "cutscene" in the place of a Codec conversation, that essensially acts as nothing more than an information dump, and a lazy way to explain information about the setting without having to actually bother working it into the world.), it is bad for a very long time. Good writers show rather than tell in their worlds. Hideo Kojima tells. A lot.
My initial playthrough took me 19 hours and thirty minutes. My second playthrough, which involved me mucking around a lot, took a shade under 10 hours, because I was skipping cutscenes. The whole game can be defeated in under 6 hours if you're good enough. This game cost me $129.99, as I live in Australia. I'm not happy about that length, at all.
My final major critique is related to this. The game is very much a sequel. Being a sequel isn't a bad thing, but the best sequels (The Dark Knight, The Empire Strikes Back, Resident Evil 4, System Shock 2) can all stand on their own as amazing games, even if you've never played the earlier games or seen the earlier films. Metal Gear Solid 4, for its 10 hours of cutscenes, spends almost every second tying up a loose end from a previous game. The piles of bizarrities from the previous games piled layer on layer (most of it coming from MGS2) were ALL explained by either Nanomachines, or a HUGE cop-out that doesn't even make sense (Liquid Ocelot sub-plot). It spends SO MUCH TIME being the conclusion to the old games in the series that it doesn't have enough time for a solid plot of it's own to develop, or to introduce any memorable new characters. If it wasn't trying so hard to be a definitive conclusion, and had the courage to leave some questions unanswered, it would have been a better game. If anything, this game needed an editor. Desperately.
I've only talked about the bad things so far. I don't really have time to cover everything, but despite all of this. It is a great game. I rated it Four and a Half stars for a reason. If you own a PS3, you should own this game.