Recently I had my SP4 replaced, and as part of my new install I used the Intel beta drivers. I had been running my 4k monitors as one off the SP4 dock and one off the SP4 port. When I connected up to my pair of 4k monitors I decided to see if the Intel HD Control Panel which installs with the beta drivers would give me more control. I connected both monitors to the Dock (mDP->DP). The control panel kept setting one of the two external monitors to match the internal panel (defaulting back to 1920x1080 for both). BUT by turning off the internal panel, I was able to run both external 4k monitors at 4k/30Hz. I tried to increase the refresh to 50Hz and was unsuccessful. When I added the SP4 panel to the mix *as a third monitor*, the external monitors dropped to 29Hz, but all three displays worked. Just thought you all might like to see this one [semi-successful] data point.