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.