Everything is stock, including skin, I don't use any add-ons other than those already installed in Libreelec.
Here are the results of the test at idle: https://imgur.com/a/8xfplsm
Are you using hdmi_enable_4k=1 in config.txt? This enables 4Kp60 but (currently) increases clocks/voltages which leads to higher temperatures.
Quote from popcornmix
The reason that is optional is it increases clocks/voltages which means higher temps. We did lower these just before LE image was made but a. Issue with WiFi meant we reverted it. The latest rpi-update does lower this again (with a fix for WiFi.).
Hopefully the next LibreELEC 9.2 alpha update (9.1.002) will have the updated firmware and thus lower temperatures with hdmi_enable_4k=1.
Have you limited your GUI size to 1080p as this will give better performance (and thus potentially lower idle temperatures) than running the Kodi GUI at 4Kp30 or 4Kp60 (Settings > System > Display > Limit GUI Size - needs settings level set to "Expert").
Can you upload your kodi.log from the RPi4 using the "upload kodi log" option in the LibreELEC Settings addon and post the link?