Which LibreELEC version are you using?
On LE10beta 4kp60 output isn't implemented yet (it's currently being worked on).
LE 9.2.6 supports 4kp60 if you add hdmi_enable_4kp60=1 to config.txt and enable "ultra hdmi deep colour" (or some similarly named option) for the HDMI port in you TV's settings.
The GPU in RPi4 is a bit underpowered for 4k rendering therefore GUI resolution is limited to HD. You can disable the limit in settings->system->display (you need to switch level to expert to see it) but then GUI navigation will be quite slow - I wouldn't recommend doing that.
Actually it's best to change the display resolution to HD (in general your TV will do better HD->4k upscaling than kodi/rpi) and enable 4k modes for video playback in the whitelist.
so long,
Hias