Nice build but I'd drop the monitor to 1440 rez for games, only the GTX1080 can reasonably power that screen and the poor R9 390X is simply going to be overpowered by all those pixels.
Suggestions:
Go for a fast 1440 display rather than 4K.
Look at either the GTX980Ti or GTX1070 for the graphics, it won't particularly matter what monitor you go for; Stock 120/144Hz or Freesync, they'll all operate as a normal, fast display if you use a combination of framerate target control and adaptive sync through the Nvidia Control Panel.
I would suggest the R9 Fury or FuryX but at the moment they're both outperformed AND undercut by the GTX980Ti.