LibreELEC are using MMAL and GBM, which is a bit of a bodge. There’s not much point in doing so from an OSMC perspective as MMAL is likely to be deprecated in the next version of Kodi. Support for other video pathways such as AMCodec has already been removed.
On LibreELEC you can play 4K video with hardware acceleration, however:
- There is no 10-bit output support
- There is no chroma switching
- HDR and HLG EOTF flagging is not supported
- There is no support for HD audio
This doesn’t make it very useful presently for playing back 4K UHD rips or commercially produced content.
The efforts to support these features are being made on the V4L2/GBM stack, rather than the MMAL stack.
You won’t get 4K support with software acceleration. I covered the limitations and what to expect from Pi 4 when the device launched in June.