I downloaded LibreELEC-RPi4.arm-10.0.0.img and installed it on my RPi4. I plugged it to the 4K TV and the image went black right after showing the LibreElec logo.
I guess it has something to do with the default resolution, since I plugged it later to a FullHD TV (with no 4k) and it worked fine.
It really doesn't have 4K native support?