I would not invest in a 4k monitor at this time.
1. A single 1080 comes up short in 4k games ... 11 out of the 25 games in Techpowerups 15 game test suite can't do 60 fps at 4k .. Out of the 4 that do, Fallout 4 gets 69 and WoW will make it even on a 970.
2. Even tho your 1080 is equipped with Display Port 1.4, there is no monitor made today that is so equipped. There are not even DP 1.4 cables. As such, without a capable cable and monitor, you can not handle all the bandwidth that the card is capable of.
3. Monitors are usually a substantial investment which peeps will hold on to for 2, 3 or even 4 builds. Once DP 1.4 monitors drop, the value of a current 4k investment will drop in the toilet so to speak.
4. With the games you play, at least some of them will be in an fps range where you can use ULMB which is a motion blur reduction technology. You can't do that on any 4k monitor. 1440p monitors rock
At this point in time, I would wait for 4k w/ DP 1.4 ... The best gaming experience currently available if on the Acrer XB271HU bmiprz and Asus Pg279Q ... IPS < 3 second lag, low **tested** response time (as opposed to the fake numbers advertised)
http://pcpartpicker.com/product/ttnG3C/acer-monitor-xb271hubmiprz
http://pcpartpicker.com/product/XvfmP6/asus-monitor-pg279q
Might wanna wait till after the holidays tho, at least for the Acer ... demand has been so high that prices have risen $100 over last few months.