Oh no. Well that's very disheartening to read. The Surface is unable to video output to a 4K display at 60HzMake sure your monitor is in MST mode, not SST. MST will make the screen appear as two half-size screens. The newest Intel drivers will then combine those into one big screen.
At 60Hz, I believe the mobile i5/i7 cannot drive more than 3200x2000 per screen, which is why single screen mode will not work.