We don’t recommend to set the GUI at 4k, keep it at 1080p (see below).
If the 1080p isn’t filling your TV screen then you have an issue with the overscan setting of your TV. That depends on your TV, if you let us know which TV you have we can help but generally you look at something like Just Scan or 1:1.
As said I would not keep the GUI at 4k while it still should work. Maybe @grahamh still want to take a look but he would need debug logs from you to check it.
We recommend people with 4K TV’s set their user interface (UI) to 1080p. Kodi’s UI is not optimized for 4K yet and this can put unnecessary demands on your device and can lead to a suboptimal picture quality, as well as potentially cause other issues.
The settings we recommend are as follows…
Settings>System>Display>Resolution> 1920x1080p
Settings>System>Display>Whitelist> (empty) *
Settings>Player>Videos>Adjust display refresh rate> On start/stop
Some televisions may also need, or benefit from, the following being set…
System>Display>Force 4:2:2 colour subsampling> (enable)
With the above settings your UI will be output in Full HD and your 4K content will be output in 4K. *Information regarding the whitelist can be found here. If you have any doubt, feel free to upload some logs so we can verify that your settings are indeed correct.