add a custom resolution of 3840x2160
On the preference menu, under "RDP" tab, set your device scale factor to 100% first, then set your desktop scaling factor to 200%.
Connect to the remove Windows using that high custom resolution.
The screen will look bloated, cut off, and ugly. Don't worry just yet.
On the side menu, click on the button "Toggle Scaled Mode"
Remmina will scale the remote session to your current windows size.
Everything will look sharp!
There is one glitch though. Remmina will not scale your cursor just right... Oh well, it is not a big deal.
But it would be nice if it gets fixed too.