The latest nightly build from here Index of / (ATM LibreELEC-RPi4.arm-9.80-nightly-20200621-7864ad7.img.gz) contains current state of RPi4 development.
Hardware H264 decoding is currently disabled by default (as it causes crashes when seeking or stopping), if you're willing to take the risk you can enable it in player settings. 4k HEVC hardware decoding works fine, though.
The KMS driver doesn't support 4k output yet, so it'll be limited to 1080p. HBR, HDR aren't implemented either and proper multichannel output is also missing ATM.
So, you have been warned, test it at your own risk.