For gaming or rendering?
i dont use SLi / xfire because i dont want the hassle of heat and im pedantic about any chance of microstutter & glitching.
Quite a few use SLI but ive only seen one build with 4x titan X. Linus techtips did one and the glitching was so bad most of the games were unplayable.
So it seems it ramps up the more cards you add. So, if gaming, id stongly advise thinking about only going with 2 cards.