I've never played the first 3 Metal Gears so I was very lost when I played MGS4. It's beutiful and so crisp, and I had a fun time with it, but completely lost in the story.
I watched the Game Trailers MGS 1/2/3 highlights to get the gist of the story, but still got confused and felt left out. The good thing about MGS4 is that it is now a Greatest Hit title so you can buy it for $30.
inFamous, which I've put about 3 hours into is also pretty fun. It's a sandbox game so if you like games like Crackdown, Spiderman 2, GTA, Saints Row, you should like infamous. The missions are fun, but they consist of a lot of FPS-like electric shooting. Its a new IP so there are no other games you need to play to understand this one, and easy controls.
I'm pretty sure there's a demo of both MGS4 and inFamous on the PSN store so just play those and see which you like better.
Personally, my favourite genre are sandbox games so I would choose inFamous over MGS4