https://www.youtube.com/watch?v=lcPATWi1bEU
short answer yes, long answer no. yeah, its going to be playable and you'll have fun as long as you're not staring at an fps display. however, you will not actually maintain a perfect 60 at all times. and witcher 3 has pretty good sli scaling. other games...not so much. if at all. i dont think 4 titan x's could make a steady 60fps on rise of the tomb raider at 4k ultra settings. and games will only get more demanding as time goes on. hopefully more optimized, but thats a bumpy road. especially in regards to sli scaling and implementation.
i find that people very much so underestimate the demands of 4k. they assume that buying the most expensive thing they can will be enough to handle whats out there, but thats just not the case. 4k gaming can be very hard on performance in "next gen" games.
also keep in mind that guy had to put a mild overclock on the cards to get that performance. while it was mostly at its 60fps cap, you will see spikes down to the 30s in certain moments of his video. its going to happen. especially at 4k you just cant expect it to NEVER drop below 60, though you likely wont notice it too much without that fps displayed.
my immediate recommendation would be if you have the money, wait until the new cards this year for the best performance.
https://www.youtube.com/watch?v=oJPh_Jqo9mQ
barring that, i would suggest 2 980 ti's instead of titan x's. a titan x is overkill, you do not need that kind of vram for gaming. a 980ti is pretty much a titan x with less vram, and are currently the best performance per dollar if you can afford it. which i assume you could.
if you're out for being able to say "hey, i have titan x sli!", go for it! i will never stop an enthusiast from making their rig as beefy as possible, overkill is often the name of the game because we like to do so. but if you're just after performance and assumed a titan x would be the best answer, a 980ti will match it for much less price.