I have a Denon AVR-X520BT, connected to my TV via the HDCP2.2/4K compliant HDMI socket.
I have Plex running on my ATV 4. Some of my movies have TrueHD or DTS-HD audio.
I don't think the ATV4 can't decode it, but looking at the ATV4 settings there's an option under audio for either "Best Quality Available" or "Dolby Digital 5.1".
If I set it to DD 5.1, then play a movie in Plex with TrueHD or DTS-HD audio, the Denon receiver lights up as "Dolby Digital" and I get surround sound. If I set the ATV4 to "Best Quality Available" and play a Plex movie with TrueHD or DTS-HD audio, the Denon lights up with "Multi Channel In". Again, I'm getting surround sound from the speakers.
So, which is the best setting?