Well your rig CPU wise is very 4K friendly as i run a similar platform but with an i7 3930k @4.2GHZ for 4K gaming. The GTX 1070 is barely 4K capable. Even running 2 GTX 1080s in sli I can't max everything in all games. When SLI is not supported the situation is even worse, usually. Point being a GTX 1070 would be the bare minimum GPU for low-meduim settings at 4K with no filtering or AA. As for you low GPU utilization I can't say. Even at 1080P with your CPU you should be capable or 90FPS or less in gaming (most titles not all). What is your CPu usage in games you can't get to the 90FPS you seek? If one or more cores is hitting 100% then your having a CPU bottleneck. If not the you may need to run DDU in safe mode and reload your drivers.