Who here is buying MGS 4 despite playing the first three?

[Magnific0]

If you want to delve deep into the story you're not too late, just go to Gamefaqs and read some plot summaries.

From Metal Gear to Metal Gear Solid 2
http://www.gamefaqs.com/console/psx/file/197909/11377

Metal Gear Solid 3
http://www.gamefaqs.com/console/ps2/file/914828/34140

There's also a nice timeline of events :

http://www.gamefaqs.com/console/psx/file/197909/43403