There isn't anything out there yet that can handle 4K gaming with settings on ultra at 120FPS. Just not possible right now. Unless you only play TF2, LFD2 and maybe some moba's. At least I haven't seen a set up that could do it yet.
At this moment 4k cannot make 120 fps. Even you did Quad TITAN also.
Basically, best option is 1440p for you . Also Quad is not good. SLI is the best one. If you can wait Radeon 300 series would be the best option for you.