X-Plane 10 starts in windowed mode by default, IIRC. You can run full screen by going to the rendering settings screen and checking the full screen checkbox.
X-Plane 10 doesn't have a way to scale the user interface for large (4k) monitors though. You might be able to artificially enlarge it by using a lower resolution. (X-Plane 11, on the other hand, has an option to scale the UI to 150% when full screen for example.)