Playing the game I use my monitor's native resolution which is FullHD (1080p).
For screenshots I use recorded NTRK files and playback the files using Nvidia DSR 4x so the resolution is 4K.
Actually my Nvidia GTX 970 is able to run the game at that resolution as well (with 60FPS, no difference to the FullHD gameplay regardless complexity of scenery, if there are FPS drops, then it's not because of the GPU), but I see no benefit from doing so.
Maybe once I can afford a 4K monitor things will be different.
In the past I've used DSR 16x setting so I could get 8K screenshots from NTRK playback, however at that resolution the FPS drops significantly (no big deal when I'm just taking screenies) and even worse, when the scenes become too complex, the 4GB RAM of my GTX 970 are not sufficient anymore and the memory overflow will lead the game suddenly to crash back to the desktop, in that case without changing the resolution back to normal, which means that I'm sitting in front of a 1080p monitor running a desktop at 4360p resolution, which simply means that you can hardly hit anything with your mouse.
Current Nvidia drivers don't support DSR 16x anymore, so I'm back to the "normal" 4K screenies.
Oh, and not to forget: Reshade is also active.
Best regards - Mike