Well, reading the Sinister arc so far, I would not say that he is one of the Four Horseman, and if so, which one? There has been no mention of such characters the titles at all, so, for now I am disregarding them.
With Gambit, I was under the impression that he was working for Apocolypse, too. I just hope that there is some relevance to his appearance rather than him just showing up again cuz all of the Ultimate fans started bitching about not having him around.

We'll see though... Either way, I'm totally ready for the next arc. With Vaughan writing, Kubert pencilling, and Gambit back with expectations for more Ultimizations of X-characters, it should be interesting as hell.
Has anyone thought that the four that Sinister killed already are going to be the Four Horseman? I mean, people complained about Marrow's death before a proper introduction and there has to be someone or way to bring her back. Why not kill four birds with one arc and have the come back to be the Ultimate Four Horseman?
Also, Sinister needs to kill 6 mutants for Apocolypse and there are 6 X-Men in pursuit... Do you think... Uh, all of them could be killed?