Hi Everyone. Issue and suggestion how to solve it, please read the whole topic.
I suggest you to make Cubase and Nuendo scalable completely (with DPI). So it will be possible to scale it from 50% dpi to 300% freely. Not strictly 100% and 200% only. Make also any Plugin window scalable to any size 
(while HiDPI is active on 4k monitors). We know, plugins will be a bit blurry then! But is still 100000x better to have a bit blurred plugins - but bigger (as it is normal on standard nonHiDPI setting), as non blurred Cubase/Nuendo (HiDPI mode). Now it is so tiny, you can hardly see what is going on there (even read the fonts) 
Just add the option to the DAW on main window + /100%/ -
and add it also to the plugin window. Like on the attached picture.
There is also issue with wrong scaling on 4K monitors. I think, the above mentioned solution can solve this too, when Menus are accidentally off the screen, watch the video here. - YouTube
I bought Cubase9, then updated to 9,5, 10, then crossgraded to Nuendo10 in hope, that this will be solved finally after a few years I am working with Steinberg products. No, it is still there. So please, solve it.
Thx a lot. Petr Fink.