MGS4 was my first MGS game, I had watched the MGS retrospectives on GameTrailers as my only preparation.
And I had no trouble following the story. Sure there were small references I didn't catch, but the story was never confusing to me.
And I absolutely loved MGS4, I have absolutely no gripes with the game, I think it is one best game of this generation so far.
I haven't played a lot of R2 yet, so I can't really speak for that, but personally, I think Motorstorm: Pacific Rift is awesome!