I have to add a remark concerning the system environments, e.g. QT_AUTO_SCREEN_SCALE_FACTOR=0. Unfortunately, these will not work as a general setting because I detected that they affect other programs. In my case, the video communication software Alfaview. After setting the above-mentioned variable, Alfaview has scaling problems on the 4K display. As a result, I have to remove the setting again to let Alfaview to work correctly.