There's a widely known issue with #Kodi, where sometimes the displayed part is only taking up 1/4th of a 4k screen.
I have the exact opposite problem, Kodi outputs 4k, but reports to the TV that it's 1080p, so the TV only shows a quarter of the actual output...
This is so weird...
And because it's tricky to reach the settings I need to change the resolution... Thankfully, the webinterface was enabled