unless the game your playing is a well known CPU resource hog then no an i5 6600k should not have trouble at 4K with a GTX 1080 bottleneck wise (the GTX 1080 may well have issues though, I run 2 of them for 4K and still can't max every game out that supports SLI or otherwise). Not everyone posting here missed this ( a lot did though). People tend to forget the higher the resolution the less CPU horse power you need to drive it, especially if you're vsync'd at 60fps or not gaming at 90-100+ FPS. Whatever your CPU can crank out with a game set to the lowest graphical settings and resolution, it can do at any resolution, assuming of course your GPU can keep up. At 4K your GPU will be the problem more times then not. Granted there are some...