The Rpi 4 has dual HDMI outputs.
Currently only HDMI_0 is initialised for audio and video.
It would be good to be able to send Video down HDMI_0 and Audio down HDMI_1.
This helps in scenarios where the TV is 4K, but the AVR is 1080p, and you still want to send PCM or DTS HD (or other lossless audio) to an AVR (which RPI 4 hardware has support for).