I agree with Blonde Haired Kid, but rather, I think it is in the future for several reasons:
A) Riku's keyblade is present, when it was said not to have existed until this game.
B) The Ansem Reports are made present, and I believe those are fairly recent too, about a year old if I'm correct.
C) The numerous keyblades. In Kingdom Hearts I, there was only two keyblades, yet in KHII, there are three. The more it progresses, the more keyblades appear.
I also think it's very possible that those three knights could be Sora, Kairi and Riku, reunited by conflict and forced to take up arms again.
As for the disinergrating keyblades, perhaps their weilders were corrupt and of weak heart, so they dissolved in the presence of a strong heart.
And finally, for the fourth knight, it could be the game's antoganist, or it could be an ally that we have yet to meet. Who is to say?