"No support for 4k output, this isn't implemented yet in the new driver. Neither is HDR output. Playing 4k HEVC files with HD output should work fine, though." - As you said this isn't working.
Atmos works might fine. Actually sounds better on the Pi4 than it does on my Nuc. So I guess better audio driver?
Only bug I had was with the scraper but that aint your fault. Probably set it up wrong.
Everything I tried worked as expected.
Even H265, 2160p (noticed it aint 4K since as you guys said :P)
So I would say a success from me. I would love HDR to test as well in 4K, with this Atmos to see if they all play nice together.
EDIT: Not sure if it's you guys or Kodi. But it mislable some Atmos as just "Dolby" but it's Atmos in the files.
The LG SJ9 also start playing Atmos when you start it. So I guess it's a minor bug.