I have a 4K monitor but I’m playing on very low settings and 1920x1080 resolution to get 60+ FPS. Every now and then when loading to a new zone my FPS drops hard, below 20 on some areas, and I can see the resolution change to 4K based on the sharpness of the graphics. In the options the resolution is still shown as 1920x1080, but if I change it to any other resolution and back the FPS goes back to 60+. Changing between maximized window and exclusive full screen seems to make no difference.
Win 10
i5-4460
24 GB RAM
R9 380