For some months I was struggling to make the 60Hz refresh rate of my 4K monitor work under Ubuntu 14.04. The graphics card I use is based on an AMD R9 280 GPU and is connected to the monitor through Display Port.

I tried different versions of AMD Catalyst driver and even with the latest 15.5 I had still the refresh rate of 30Hz.

Solution was in the monitor settings: the default stream version for Display Port was 1.1, changing it to 1.2 enabled the 60Hz mode.