I recently purchased a ASUS 4k Monitor, I hooked it up via hdmi and it pretty much changed my computer using experience, I use spectacle so I can keyboard shortcut windows to corners of the screen. Very shortly after I realized the refresh rate on the monitor was a bit slow, after further investigation I realized it was 30Hrz, or 1/2 of what the monitor and late 2013 Macbook Pro are capable of running. These are the steps I took to get 60 hrz:
Buy a 1.2 mini display cord, I used this one
Monitor menu > system seetup > display port stream 1.2
Set computer to 60hrz, this process for me involved going to system settings and setting it back to "default for display".
If you have any questions hit me up on twitter @tyler_merry or shoot me an email at tymerry@gmail