I just purchased X-Plane and have the same problem as others using UHD. I have a Lenovo Y50-70 laptop with 4k monitor,16 GB ram, and a second Dell U2715H 2560X1440 monitor I would like to run the program in full screen. The display cuts off some of the instrument panel at full screen; however I can scroll the full view with the arrow keys. Unfortunately, the biggest issue is not being able to control any of the planes motion with my mouse. This is very frustrating as I already purchased the program and am unable to use it. The mouse box only appear about one square inch, regardless of varying screen resolutions or text size, making it impossible to fly the plane. I also have the same problem as previously described with the tiny dialogue boxes, which I can at least see well enough on the 4k monitor to adjust settings with a magnifying glass. I tried changing the text size down to 100 as mentioned; however, the settings box became so small it's unusable.
Waiving the white flag.