Update on wake from sleep 4k monitor:
Only 4k monitor connected >> same result. Will not always wake from sleep.
Tried adding a second NVIDIA GTX 980TI and connect one monitor to each of the cards, same result. Low res monitor wakes, 4k monitor sometimes.
And this is when connected ont the DP, HDMI is fine (except for the 'snow' sometimes after wake or res switch).
Turning off CSM in BIOS >> no change.
Windows always fine, so related to OS X or NVIDIA drives for OS X.
This is an issue many users report, it is not related to just one brand of 4k monitor or motherboard brand.
I do hope this get fixed someday.....