You can create new modes for xrandr to fun at a higher frequency. On linux (ubuntu 16.04), I could only get my rx480 to put out 50Hz at UHD stably. At 60Hz the screen flickers. On windows, however, the rx480 puts out 4k 60Hz 4:4:4 without an issue - so it must be an issue with the linux amdgpupro driver.
Anyway, here's how I enabled 50Hz:
xrandr --newmode "3840x2160_50" 445.008 3840 3982 4027 4064 2160 2170 2180 2190 +hsync +vsync && xrandr --addmode HDMI-A-0 3840x2160_50 && xrandr --output HDMI-A-0 --mode 3840x2160_50
Remember to substitute HDMI-A-0 for your connected device.
Also to note, this command isn't persistent, meaning you'll be at 30Hz again every time you reboot.