What are you planning to do with that 4k-monitor ?
One thing you must realize: if you keep doing the same stuff, watching the same websites, watching the same photos, using the same applications, using the same source material: EVERYTHING WILL BE TWICE AS SMALL.
As compared to using a same-size monitor with 1080p resolution. Windows will show you all its stuff twice as small. Icons on your desktop, your file-browser, all applications will be tiny with tiny letters. Now you can enable "scaling" but that just means everything gets blown up by 2, and the result will be just as pixilated as a 1080p screen at that size. Browsers is even worse. You can pick larger fonts, but that only works to a certain extend. Often it gives problems with overlapping text and things going wrong. And every picture, every flash-object every movie in your web-browser will be twice as small.
Games look better at 4k, sure. But it takes 3x the horsepower in your GPU to drive games at the same settings. Games that run at ultra on a 1080 at 60 fps will run at 20 fps on your 4k-monitor. And even if buy 2x gtx1080 in SLI, that power could be used to make your game look better at 1080p. More polygons, more objects, more NPCs on screen. Better AA. Better shadows, better HBAO+. More foilage. You name it. The endresult of such a game 1080p will look better than at 4k with medium settings.
So what is 4k good for ?
Anything that has 4k-source material. That could be 4k-movies. Or large high-res pictures. Those are more likely if you make movies yourself, or are a photographer. But as a consumer, I think there is very little stuff you can do yet that requires a 4k-monitor.
And now all the enthousiast can tell us how 4k changed their lives, and they couldn't live without it. Maybe in a few years (when movies, pictures, applications and websites are made for 4k). But for now, 4k-monitors are about as useful as 3D-tvs.