We have engineering workstations that we just upgraded with dual 4K Samsung U28850R monitors. They works great, once the system is booted, but we are having problems getting past BIOS on a regular basis. Typical voodoo magic required is something like: Power monitors off, boot system, wait 10 minutes, power monitor on, login to Windows. If the monitors are on, we never seen a BIOS message and the system just hangs.
Unfortunately, the “voodoo” works about 1/2 the time, often requiring several retries.
Both monitors are connected via DLP (technically one mini-dlp cable, one full dlp cable - mostly because we had those available). If I replace one of the monitors with a standard definition one, everything boots just fine 100% of the time.
I have a half-dozen of these setups, all behave the same.