Sorry for the misleading title, the issue has changed since I posted. Mods, feel free to adjust the title.
Running 10.11.6, on i5-4690K, with SMBIOS iMac 14,2, board is Gigabyte G1 Sniper M5, connected to a Dell P2415Q via onboard DP1.2.
I got the HD 4600 to output 4K @60Hz as suggested here...