Tod, thank you for the info about Brightsign.
For anyone who has a similar problem—trying to play your own edited 4K or HD MP4 videos through an A/V receiver for improved audio (vs. playing through a TV), I ended up purchasing the Dune-HD Magic 4K Plus. I’m very happy with this unit.
I connected the Dune-HD via HDMI out to the Marantz ‘media player’ HDMI in. The Dune Magic 4K instantly finds and plays the attached USB SSD drive. 4K playback is … 4K playback, and the audio through the Marantz is obviously better than connecting through the TV.
I did have one issue initially: when playing a video there was a 2-second video and audio blackout about 1-second into the video. Dune suggested resetting the video mode from ‘Auto’ to 4K 2160p 29Hz in setup. That change solved the initial black-screen blink. Videos now play immediately.
The Magic 4K Plus was less than $140 on Amazon.