Yeah, as others have said, it goes 3, 1, 2, 4 in terms of the chronological order of the series' events. Funny enough, just like the MGS series. That said, the storyline is nowhere near as engrossing as most other games.
There's never more than a handful of characters and even though the third game in the series takes place before all others, its not hard to piece together and/or follow the events playing them in release order. Which brings up, yet again, another good point that was already mentioned... if you play them in release order, you'll come to appreciate how the series has evolved from game to game.
But yeah, you shouldn't have trouble following the story even if you played them in 4, 3, 2, 1 order, heh.