I have a Mid-2015 15" Retina Macbook Pro running the latest macOS Sierra version with the AMD Radeon R9 M370X dedicated GPU and I have been trying to get it hooked up to my Crossover 324K 4K monitor without success.
The symptom I am experiencing is that I will plug a the DisplayPort end of my DP to mDP cable into the DisplayPort connector of the monitor and the Mini DisplayPort end into any of the two Thunderbolt 2 ports on the Macbook. The screen resolution scaling switched back to default on the macbook and I see the external monitor light up and sometimes display a picture for a fraction of a second and then it goes right to no signal and the resolution changes back to what it was on the macbook as if no external display was plugged in.
The monitor has an option in the settings to toggle between DisplayPort 1.1 and 1.2 modes. I can get 4k at 30 hertz to run if I set the option to DP 1.1 but the monitor doesn't work at all if I set it to DP 1.2.
Additionally, everything works flawless on windows with the latest AMD drivers so I suspect something about mac os is to blame. I can have the monitor running at 4k, 60 hertz and DisplayPort 1.2 on windows 10 just fine... The last oddity I have discovered is that I can boot up the Macbook into osx with the lid closed and the monitor plugged in (in DP 1.2 mode) up until the login screen which renders without retina scaling. As soon as I type in my password to decrypt the FileVault disk and login, the display goes blank and switched back to the internal monitor so I'm basically stumped...