Should I get Metal Gear Solid 4 or just wait for Metal Gear Rising?

I don't know the story of the Metal Gear Saga so much I mean I have played MGS with my my cousin over the years I still remember when I told him in MGS1 that although he will use stealth cameo in snow he will still be detected by dogs (sniff everything), I played MGS2 the demo only so I didn't get the story and MGS3 I only saw a couple of trailers and the boss fights. So basically, I don't know the Metal Gear Story so much can someone please explain it to me in a brief understandable paragraph? Because before getting into a game I would like to know the story for example Mafia II (brilliant story), God of War series, Ratchet and Clank played through all of the games. Or should I just wait for MGS Rising and experience a new story? Please help me choose