I thought it important to share with you issues re 4K screens.
I recently purchased a Dell xps 15 and experienced the same issues others are having, after speaking with Filemaker I was able to create a work around.
Filemaker said to reduce the scale to 100% and then work on solutions at 200% that is fine until you need to open another application and then you need a magnifying glass to see.
I set the scale to 200% and it worked and still had the benefits of a 4K screen, very happy.
Note this is a work around for my PC using the built in screen not an extension screen so not sure how that will work but expect it should.
The attach PDF gives you screen shots and instructions on the work around til Filemaker resolve the issue
Let me know how you got on,