If you didn't have your graphics cards yet I'd recommend switching to NVidia and get the upcoming G-Sync monitor (Asus 2560x1440 120Hz, non-IPS).
4K is a waste of money currently IMO when 2560x1440 is pretty similar for cheaper, with more proven technology.
You don't have to game at those resolutions though (3x4K would be idiotic anyway) so 3x1080p makes far more sense regardless of the native resolution, but use the native resolution for non-gaming. If you are sticking with Crossfire I recommend considering an IPS panel (60Hz) instead as the colour and viewing angles are better. When you turn a monitor on an angle (Eyefinity) you don't want issues there.
Question is stated in the title, price is not an issue
This does not exist. Yet. You could build a setup 2560x1440 120 hz+. Also you would need ALOT of GPU and CPU. A whole lot.
What if i told you I have 4 titans in my amazon cart right now!
That is a lot of gpus, hopefully your already on 2011, might as well be on a WS grade board also. I think evga is going to release a 780ti with 6gb vram, i think. Otherwise R9290x2 is probably the way to go right now, never the less there still is not a "4k" 120hz monitor out. I agree with @photonboy 4k is a fools errand right now, especially for gaming.
Don't get 4 Titans, such a waste. 290x's or a 295x2 are better for high res gaming, and like mentioned a 780ti 6GB version is coming out soon probably. All of those outperform a Titan for less money. Titan is NOT a gaming card.
Also, like mentioned, there is no 120hz 4K monitor, and won't be for some time. Even if there was, there would be nothing out there that could run it. It takes 2 290x's to pull close to 60fps in games at 4K, so you would need twice that for 120hz @ 4k. There is no card solution that powerful.