Yeah I wouldn't fancy giving MGS 4 a shot without the others. That game is pure nostalgia driven.

MGS 2 is pretty fresh up until the cutscene before the Metal Gear fight. It'll feel like a brand new game, and the only thing you won't get up until that point is the subtle shit about Pliskin.

Metal Gear Solid 1 if you don't mind the graphics. That game is bonified AMAZING for it's time.