It's just scaling it back down to 1080 on you.
Unless it's letting you pan around the full 4k desktop, then that Virtual Desktop and part of your gpu software
The Scaling would be built into the monitor usually but it can never show you more then its native resolution.
If you go tot he edge of the screen and then the desktop starts panning, showing you more of the desktop, that is the Software. i think AMD calls it ReLive now?
Maybe, I'm not really an AMD GPU person but it sounds like its right.