All four Sith? Lets see...
Vader, with his decreased speed once encased in his life-sustaining armor, and that suit's clear weakness against Sith lightning (it did destroy Vader, after all) would quickly fall to either Tyrannus or Sidious. I could forsee Dooku having some trouble with Maul, with two radically different combat styles meeting. Dooku would have the experience edge, but Maul would have the emotional edge (something that Sith seem to draw a great deal of power from) as he was fanatical about his hatred of Jedi. Maul would not take a former Jedi-turned Sith Lord lightly, and would likely be quite enraged by the concept. I'm not sure how that would turn out, but I'm confident that if Sidious did not destroy them both in the midst of their battle, he could easily handle the victor.
So, Vader goes down first, and Sidious comes out on top.