I have a 4k Dell XPS 13 and Gimp has been miserable to use.
All the controls have been so small as to make clicking on them almost impossible.
I've tried different themes within Gimp and adjusting setting. Nothing helped much.
Then I adjusted the Dell resolution to 1920x1080 and Gimp looked normal.
... I adjusted Dell back to recommended 4k
...... Closed Gimp
......... Opened Gimp again
............ And got this
Looks normal to me. Hope is sticks
***Caution - I am positive I tried this before with no positive result but wouldn't swear to it. So give it a few tries.