The download links for Pi 4 and 5 give a 404 because the URL still says "arm" instead of "aarch64", but once I found the correct link, the build for Pi 4 seemed to work fine. I had to manually install a couple of the LibreELEC add-ons, namely InputStream Adaptive and Network Tools, because the compatible versions don't come up in the repository yet, but that's pretty expected with a beta.