here's how i got passed 30hz in 4k on my Sierra hackintosh.
tl;dr - I upgraded the firmware on my CAC-1070 DP/HDMI converter, installed switchresx, defined a custom resolution @ 60hz, then (most importantly) turned my 4k screen off and on
hackintosh Sierra 10.12.6
geforce gtx 1070 - most updated web drivers (378.05.05.25f04)
primary display - Sceptre U425CV-UMC (budget 4k), connected via CAC-1070 DP/HDMI adapter
secondary display - Lenovo LI2364D, connected via HDMI
so I've been struggling getting 2560x1440 @ 60hz. Everythime i tried anything over 1080, my main screen would generally just go blank and become unusable even after reboots. i'd have to wipe display plists and/or shuffle around hdmi connectors & ports on the back of the computer. It worked fine in Windows, so it just didn't make any sense why it didn't on my mac install.....
After much frustration, trial and error, this is what finally worked for me
0. modify SMBIOS to have the OS (snowflake warning) self-identify as an iMac17,1 (not sure if this step is even necessary)
1. upgraded the firmware on the cac-1070 connector. follow instructions here
2. install switchresx; do not reboot. load up a custom resolution of 3840x2160 using the 'simplified settings' of CVT-RB. if the scan rate doesn't auto-populate, manually enter 60hz. save settings. reboot.
3. so after this reboot, my primary display went completely dark and the OS didn't even recognize it. the simplest yet most important step comes here. turn off / turn on the display. it was recognized by the OS at the default 4k / 30hz, so i then switched to the 4k/60hz custom setting in switchresx. from there i repeated step 2 to implement 2560x1440 @ 60hz.
4. Done, son.
so now, the only thing that's a bit annoying is that i have to turn off / turn on the display every time i boot into the hackintosh. The OS still has a hard time seeing the 4k display even from a cold boot. a little strange but at this point, i'll take it.
anyway, hope this helps others who have been struggling with the same....now if you'll excuse me, i'm leaving to bask in the 4k/60hz glory....