Cloning a 4K panel to a 1080p monitor
Recently I found a nice way to solve this. Im answering on my own question so everybody having the issue can read it. I now use a 4K screen getting the full 3840x2160 px output from my GTX 970 and a second 1080 px display displaying the same content. So I mirrored the 4K display with downscaling to a display with less resolution.
Having a NVIDIA graphics card there is no need for any additional software or even hardware.
HERE'S HOW: Just right click on the desktop and select NVIDIA Control Panel. Go to the 3D Settings and click Manage 3D Settings. Scroll down to DSR Factors and set the setting to 4.00x if you want to display 4K footage on a 1080 px display. Now you go to the windows display settings and set the resolution on the 1080 px display to 3840x2160 px. Of course this does not provide better quality on the 1080 px display. It might be worse but quality is not the reason why i have been doing this. Also I dont know if there is a equivalent solution for an AMD graphics card.
I'm not entirely sure that this can be achieved with built in software, the "cloned" output expects to put out the exact same picture on multiple ports.
This means you either need
- to have it at 1080p and let the 4k screen do upscaling or, if you still need the 4k resolution,
- you need to downscale the image for the 1080p monitor.
For the downscaling solution something like a hardware 4k to 1080p converter box would do the trick. Something like this 4k to 1080 up/downscaler.
These kinds of bits of hardware should not be significantly expensive, but I think it is the only way it is going to work in practice.