When you plug in one of those 4K 60 Hz monitors, you're probably using MST - most of them are designed to appear to the host PC as two separate monitors daisychained over DisplayPort MST, with some extra metadata to tell the OS they're actually two halves of the same monitor, and OS X supports this on newer hardware. Which means Mac OS X has code somewhere to detect if you're trying to actually daisychain non-Apple monitors and disable MST.