When I try to play on in windowed mode on my 4K monitor (1920 x 1600) a lot of the UI is not scaled properly and in some cases you cannot even use the UI to its full extent.
For example, when I open up the Skill UI with the “S” key, I am only seeing the center potion of the UI (exactly 1920 x 1600 out of a possible 3840 x 1600). In other words, 25% of the left and 25% of the right of the UI is not displaying.