I need to use the Windows->Developer Tools->Widget Reflector to scale the editor UI (unless there is another way to make the fonts bigger). However, when I up the “Application Scale”, the UI gets bigger, but now I am unable to select things in the 3d viewport. The bigger the scale, the more ‘off’ the 3d selection is.
I’m using the binary 4.7.
Hello ZealUE4,
After doing a bit of digging I found that this is a known issue (UE-7044) and I found that UI scaling is not being supported. The widget reflector is being used as an easy way for us to test various features in the engine, but is not intended for normal use. However, I have submitted a feature request ( UE-12607) to allow users for change the font/icon size for editor. I will provide updates with any pertinent information as it becomes available. I hope this helps. Thank you for your time and information.
Here is a link to the original post:
Make it a great day
Hi , are there any news about this subject? users with an uhd display have to work with a bad dpi-scaled editor for now and there is still no such feature to scale the editors UI 
Hello BeeHive,
I went ahead and double checked this report for you and found that the status has not been updated to fix as of yet. I will be sure to bump up the community interest for this issue.
Make it a great day
Close the Engine. Right click on the EpicGamesLauncher > Properties > Compatibility. Check the box for “Override high DPI scaling behavior.” Set “Scaling performed by:” to “System”. This works for me across resolutions/ windows scaling.