I got the RPi 4 and messed with it for a while with no luck. I tried to get it working with Archlinux (my preferred distro) but ran into lots of issues, more with Arch Arm specifically, not with the originally raised concern of HDMI support.
In the meantime I pivoted to just running a 50 ft HDMI cable from my Mac Studio to my Denon as I wanted to have that for several reasons anyway. Roon unfortunately has been quite annoying and flaky as fuck for me (audio outputs keep disappearing / resetting and whether it recognizes the Denon as a stereo output or 7.1 is totally random) so it turns out that it's also not a good solution for me to get multichannel audio.
I'm gonna try the RPi 4 again with a more dedicated distro (like LibreELEC) soon. I'll update with my success or failure. I realize that multichannel lossless audio setup is not a common thing, but damn I didn't expect it to be this hard to get working.