You need HQ HDMI 2.0 cable for 4K@60Hz
I suggest you to reinstall your drivers.
1.Delete C:\AMD folder
This about an ASUS AMD RX 480 graphics card running on Windows 7 x64.
I have this same issue, but with a slightly different twist. My display is a Best Buy Insignia 32" 1080p TV (120Hz model). Shows up as model NS-32E859A11. I have uninstalled the drivers completely using DDU and reinstalled and still no luck getting 1920x1080 at 60hz. The driver says the display can only do 1920x1080 at 30hz interlaced. Custom resolution option won't let me select anything higher. The highest it will allow at 60hz is 1280x768. I can plug into my Samsung Display with DVI and it finds the proper 1680x1050 resolution at 60hz with no problem. So my thought is the driver is mis-reading or screwing up the EDID from the Best Buy display. I have tried a DVI to HDMI cable to thinking there may be a port or cable issue, but same restriction exists. Please let me know what I can do to get the proper resolution and refresh rate for my display.
Plugging in my Raspberry Pie finds the correct resolution on my display. Using Ubuntu 16.04 with AMD drivers in the same computer finds 1920x1080 at 60hz. It's only the AMD Windows 7 install that has the problem. I even installed on Fresh Windows 7 install and the same issue occurred.
My system has an AMD 8320e processor with 8GB of memory. Runnning on an ASUS M5A78L-USB3. The Bios is at the latest version. This same computer previously had an Nvidia GTX 650 and that allowed 1920x1080 at 60hz and never had an issue. I have tried AMD drivers from 16.5 to 17.3.2 optional driver all had the same problem.