There are several issues.
First, Greenshot's default UI does not scale with Win10's desktop scaling factor. When a monitor is in 4k res and the scaling factor is 200%, Greenshot's context menu and settings window fail to scale properly, resulting in tiny texts, impossible to read. This problem exists regardless of whether I use NVIDIA DSR to scale down 4k signals to my monitor at 1080p OR using an actual 4k display without NVIDIA DSR.
This above problem is potentially fixed by enabling High DPI Scaling Override and set to "System" or "System(enhanced)". However...
Second problem, when High DPI Scaling Override is used, regional screenshot capture does not function properly, the capturing canvas is been zoomed in by 200%, resulting in only 1/4 of the desired image that can be captured. This problem may be caused by NVIDIA DSR as I have not had time to test with my 4k monitor
Windows 10, 3 monitors all using NVIDIA Dynamic Super Resolution to simulate 4k resolution while the display's native resolution is 1080p.