Hi
This is a good question. A lot of TVs have a better scaler than Kodi’s scaler, so it’s better to output the GUI at 1080p.
If you enable Adjust Refresh Rate then your device will automatically switch to 4K resolution when you play back 4K content.
Make sure your audio is configured correctly to take full advantage of passthrough if you have a receiver and that should be that
Enjoy the new device!
Sam