I have used RP3B+ and LibreELEC with the Mythtv PVR addon for years. It works just fine for playing MPEG2 HD TV recordings made on my backend using a HDHomerun networked tuner.
I wanted to duplicate this on a RP4 which I understand is only really supported by Libreelec 9.2.0 at this point. My RP4 is connected to a AV receiver which supports 4K via it's inputs as 3840x2160 30P. I have not tested plugging directly into the TV itself which can do 3840x2160 60P.
So when I use 9.2.0 on the RP3B+ I only have to set the IP address for my mythtv backend to get perfect playback on my recordings. If I do this on a RP4 I have horrible lipsync problems. Maybe 10-20 seconds behind.
Is this a settings problem? or do I need to wait for RP4 support to mature??
Jim A
UPDATE: Since all my content is 1080i or 720P, I forced the System -> Display resolution to 1920x1080p, Refresh rate to 60fps, and Audio - Allow passthrough to on.
This seems to fix the lipsync problem.