I have an AOC U2879VF monitor which supports DP 1.2 and HDMI 2.0, but my R9 390 doesn't support HDMI 2.0, so my only option to get 4K@60Hz is DP. I have been using DP and actually gaming at 4k@60Hz for many months. Since my last reboot, however, I am no longer getting the option to set my desktop resolution to 4K; the highest option is 2560x1440.
I am using Crimson 16.9.1.
This had previously happened to me, and I posted on reddit; the post has full details about my setup : https://www.reddit.com/r/AMDHelp/comments/4yt6xd/4k_resolution_no_longer_supported_on_running/
After a few days, the options had magically returned....
The things that have changed from back then is that I have just today added 16GB more of RAM, which, on my motherboard, the DRAM speed needs to be dropped to 1600MHz to work. I have also updated to the latest Crimson version.
Other than waiting for another miracle, what else can I try? I tried adding 3840x2160@60Hz as a custom resolution, but it didn't work - I'd get some wonky resolution.
....aaaand it just fixed itself, just like last time, without me doing anything. Go figure...