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I'll chime in here because I've been wrestling with the same issues after wanting to upgrade to 4k and share my experiences with the Gigabyte G1 Radeon RX 470 on Xorg and the default Arch kernel.
First I bought a Samsung UN40KU6290 and never got it past 2160p30. I used the AMDGPU driver because the PRO driver looked liked a headache and I might as well go fir a GeForce 1060 if I'm going to do proprietary stuff, my experience with Nvidia on my laptop was relatively drama free (other then bumblebee). I tried HDMI directly, a display port to HDMI adapter and neither worked at 60 Hz. I screwed around with some custom modelines as well, no luck. If I remember correctly, Wayland displayed a 59 Hz option, but the display never cooperated.
I wound up returning the UN40KU6290 due to lack of power management (had to manually turn the TV on/off with the remote) and the corners were hard to see as it was just too big for my desk.
I then bought a LG 27UD58 and plugged it in using Displayport and everything just worked at 2160p60. One strange thing is that the GNOME Displays control panel screen shows the wrong size, so somewhere GNOME is screwing up the EDID data but it doesn't seem to matter. The display turns on and off as I'd expect and looks great.
I played a little bit of Dota 2 at 3840x2160 and the RX 470 managed 50-100 FPS on the 3/4 graphics setting and 30-55 FPS on 4/4 "Best looking" setting. I didn't know what to expect, but this seemed a little faster then expected. I assume (hope) this will get better as AMDGPU work continues and stuff like Freesync is added.
Now if only I could get per monitor font scaling so I can use my existing displays with the new one...
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