Connecting my LG 4K monitor to a generic Windows10 PC prevented it from starting. Only its Fujitsu boot message showed up, offering the option to go into BIOS setup. Which didn’t even work either. Once the machine started while being connected to a DVI monitor it happily used the 4k screen – even in its intended resolution.
PIN 20 of the Display Port seems to cary 3V from both ends, which prevents the system from starting since the graphics card gets utterly confused.
The simple workaround was to use a Display Port to HDMI adapter. That cable does not cary any 3V from both ends. I only get 1920 resolution, but thats fine.