Okay, there is no fourth knight but there is a fourth figure.
The first knight we see is uber tall and holding a big ass keyblade and we see something behind him but then it vanishes. The next knight is uber short and appears to be holding some kind of amped up Jungle King from the first game back handed(deffinitely not the same keyblade but it looks similair). The third knight (the one on the left) is taller than the 2nd knight but still shorter than the first knight and is holding a keyblade that looks like Oblivion.
Now, the three main keyblades we see are Mickey's, they keyblade form the world of darkness, Sora's in the middle, the keyblade from the world of light, and then a keyblade thats handle looks like Oblivion, Oathkeeper, Riku's Soul Eater sword (which is still NOT a keyblade) with an angel wing combined. The first knight takes Sora's sword, the short knight takes the new keyblade, and the third knight takes Mickey's keyblade.
Now, I think the one on the right is Kairi if she's even there. I would say the middle one is Riku and the third knight is Sora (that is if the three of them are Sora, Riku, and Kairi).
Now, first off, if any one is going to be holding Oblivion it isn't going to be Kairi. Oblivion's original name was Riku's Keyblade and Oathkeeper's original name was Kairi's keyblade.
However, I will acknowledge that Knight number 3 deffinitely has some cleavage space in their armor so they're probably a woman, if not that armor is odd to say the least.
I think that these knights are new characters we've never seen before and probably from the past, not the future. Xehanort is probably the first unknown (memories of Xehanort). The lost two.... got me there....
Chasers were probably people who sought after keyblades for power and the keyblade war was probably just a war between people fighting for the keyblades. I think it's cool though that the keyblades make the crossroads. Because it looks like a battlefield it suggests that the keyblade war occured inbetween all worlds.
Tetsuya Nomura is screwing with our heads again... and I love it so....