actually i wouldnt suggest anyone to buy this game and jump into the bandwagon (like i said before)
MGS4 does poorly to introduce new gamers just because it is a conclusion to the story and since this game is heavily story based game, the characters, climaxes and the conclusion wont have much impact than it is to those fanboys, and those who have played the previous games (yes there is a difference)
there is just too much information being told, its like saying your going to watch LOTR Return of the King without watching the first 2 (without being familiar to the story at all), you wouldnt know what the fucking deal is why they climbing a mountain to burn a measely ring...
play the previous 3 and Portable Ops if you want to get a full experience on MGS (that is if you can put up with hours, and hours of story telling, the previous MGS games does have lengthy cut scenes and dialogues as well...
and
to compare Portable Ops to GTA4
Portable Ops is based on MGS3 gameplay and engine, which essentially is based on the previous MGS games, to simply put, Portable Ops is indeed an MGS game...
MGS4 is no different, except this time, players are given the option to take the more action oriented route, rather than sneak around.. but essentially, all MGS games are pretty much similar in gamplay, with the exception of Ac!d of course..
after finishing MGS4, now i realize how important a canon Portable Ops is to the storyline..