https://bugs.freedesktop.org/show_bug.cgi?id=98333
--- Comment #16 from Daniel J Blueman daniel@quora.org --- I am experiencing the same 4K @ 30Hz limit with my RX460 and LG 27UD88-W monitor via a Highspeed HDMI cable. I have enabled HDMI DeepColour on the monitor, needed for the monitor to advertise 4K @ 60Hz.
Kernel is stock 4.9.11 on ElementaryOS (Ubuntu 16.04) with the HWE stack.