I have two monitors, 1 is a 4K and the other is not. When hooking up to the dock I could only get one to work at a time. Turns out you cannot use two HDMI connections. I left the HDMI connection in my 4K and put a VGA to mini display port on the other. Now they both work. I think there are others out there with this problem? If so, here's a solution. I am able to use the full 4K resolution too. It's so nice to have both monitors working! I can also use the monitor on the surface book. Now, back to work!