Well, I do not know what display you use and how it is connected as you have supplied the minimum information possible.
For example I can't get the max res at max refresh rate on my 4K Dell (3214) connected with DP1.2 to an XFX R7 260X with Windows 7 Pro 64. But depending on the driver versions I get different results.
WHQL 13.12 uses half the Display with 1920x2160@60Hz
13.30 Beta (The best I tried yet and still use) 3840x2160@30Hz
14.1 Beta displays the right half of 3840x2160 but my second screen (2713 with DVI) does not display anything.
Not tried the 14.2 yet.
The advice I can give you, try to find the right drivers which give you the best results, may not be 100%
On my second rig with 8.1 the display shows 3840x2160@60, but this has nVidia Hardware and requires a little bit of fiddling with the current WHQL Drivers until it does. It uses to work ok (without fiddling) with the previous WHQL