Depends on the performance level you are after. With that much investment I would expect nothing less then 60FPS at 11520 x 2160. Just not feasible right now.
4 GTX980 would maybe pump out around 35FPS on Ultra, or less, not sure when it would run out VRAM. Might be able to push 60FPS by sacrificing some quality settings.
I would almost opt for the R9-290X at 8GB per card to try it.
Give it another 2 or 3 generations of GPU hardware, and 4K will be reasonable with one GPU. When that happens, a set of 3 or 4 could easily run 3 4K Monitors.