Metal Gear Solid 4 is a terrible game. It does have good gameplay for the 1st two chapters and then the game throws that away. But the first two chapters aren't even enough. Chapter 1 takes place on a Battlefield and almost sells what you thought were the mechanics in this game, then realize the Sneaking on the battlefield thing is half assed as this fraction stuff means noting. Then the 2nd chapter, which is shorter, happens and you are just forced into the rebel fraction. This level does have an awesome part of Sneaking into a base that is under siege, thats what this game needed more about. Then you have a real boring follow the trail part and chapter 2 is over. Next you have the incredibly poor and boring chapter 3. The terrible chapter 4. And lastly the terrible chapter 5 that is only 3 area's basically.
Now compare this to MGS3. That game felt long and you were sneaking in different places, with some areas being quite large. Ton's of variety and the game felt like it explored it's mechanics. It was satisfactory with it's gameplay. A great game, with a good story. A story that was easy to follow and intriguing as it had set up an interesting plot with the Boss. The dynamics, you wanted to see where it was going. The game had fun bosses as well. All in all it was a great game.
MGS4 though does not feel long. Thanks to the ratio of cutscenes you feel like you are watching more and the gameplay segments don't feel that long. There is variety in the game , but stuff like chapter 3 just sucks and I myself never felt the Battlefield Sneaking mechanics were explored at all and that what I thought MGS4 was going to bring to the table. The story is bad and takes a crap on characters. It's convluted and just feels like pure wankry. Add in the fact of magic nanomachines and you have a plot that can not carry this game like MGS3. The bosses are all pretty terrible and boring, few have any personality or do anything unique.