[zlynx]

I have a very similar setup: a Dell m3800 and a 4K external monitor.

Wayland, or rather gnome-shell, seems to do EDID detection a bit differently than Xorg. Getting a higher quality DisplayPort cable fixed it for me. The earlier cable did OK at 30Hz but freaked out trying for 60 Hz. X11 must retry more times, because it would usually manage to make it work for a while on the bad cable, although it still could randomly flicker off and reset the connection.

[alphadevx]

That's what is the issue with my set-up: EDID detection fails, so it defaults back to 1024x768 max resolution, with no way (that I can figure out) to manually override the resolution and refresh rate on Wayland. Tested with Fedora 27 yesterday after upgrading, still not working so back to X11 for me :-(