Mac Mini Late 2014 4K 60hz
The CPU in the Mac Mini late 2014 is not capable of driving a display at 4K at 60Hz.

This Intel Quick Reference Guide, under the "Display and Audio Features Comparison" section, lists the Intel Iris 5100 (the graphics chip in the Mini) in the second column. Cross referencing this with "Maximum Display Resolution" in the first row, you see that the H-series CPUs combined with the Iris 5100 can drive a 4K display at 60Hz, but the U-series that is in the Mini cannot.