Extra Large is only good for up to about 200% DPI. 4K needs a DPI of around 300%. You can start Spine with a UI scale parameter using either
-s or `
Spine.exe -s 1.5
args -s 1.5
./Spine.sh -s 1.5
Of course you can use any value in place of 1.5. 1 means 100% (the default size), 2 is twice as large, 0.5 is half as large, etc. Technically this scale parameter is marked as experimental, but it should work fine.