Can you please try if this build works LibreELEC-RPi4.arm-9.2-devel-20200817130411-96056bf.tar ?
Just copy it to the Update Windows share and then reboot.
See also this github issue for more info about the bug [BUG] · Issue #4523 · LibreELEC/LibreELEC.tv · GitHub
This image has successfully updated my RPi4, there is a picture, everything is fine. I use 1080p resolution