4k isn't going to be really mainstream still for a few years, you are just taxing your gfx card for no reason. 2k will continue to be the sweet spot for some time yet. Whatever you get, 2k or 4k, make sure it has adaptive sync (gsync for nvidia or freesync for ATI) and at least 144hz. A 2k 144hz monitor will enhance your gaming experience, a 60hz 4k will not, but if you can afford a 144hz 4k monitor and the graphics power needed to run it, more power to ya.
The point here is that anyone who seriously cares about their gaming, NEEDS a 144hz adaptive sync monitor, to invest in an upgrade and not do that, is to throw away your money on something not significantly different to what you had. It not only transforms your gaming and gives the impression of a 30% increase in graphics power (due to no vsync issues or tearing, and ZERO input lag), it also makes everything you do on the pc seem like sipping Bailey's it'll be so smooth. You can read web pages while scrolling, it really is nuts. Everyone who sees my rig is blown away not by the FPS, but the smoothness with which they are delivered. And I have one of the cheapest gsync monitors out there.