Just an FYI for you folks; yes, Zero no Kiseki is not a prequel, but neither is it an entirely new story. From what my sources keep telling me, (as in, people who imported and played the game,) it's.. not exactly a direct sequel, but it's still very much related. Think of it as another installment in a continuing series; you have a somewhat new cast,
(oh, by the way, Estelle and Joshua are in this one too, just no longer the leads,) a new setting, and a new plot-focus, but it also continues a number of reoccurring plot-points from first three games. In short, while you could enjoy it on it's own, you'll get a lot more out of it you play the original Sora no Kiseki trilogy, and doing so is pretty much recommended.
ADMIN EDIT FOR REAL: Wow, messed up there.