"What about 4 Titans" Do you know anything at all about multi-GPU scaling? Most games are not engineered to take advantage of multiple video cards, let alone four of them. In fact, in modern games even AAA developers don't really touch SLI/Crossfire. Nvidia, for their GTX 1080, does not officially support more than 2-way SLI. Want to know why? Poor scaling.
"What about 60fps?" If you run DOOM on ultra graphics (not maxed, ultra -- Nightmare is maxed) then you can get 4K 40-60fps. You won't get 100fps though. Those video cards don't exist yet. You'll have to wait for the video cards with HBM 2.