I have a 4k capable LG OLED TV (HDCP 2.2) with a 4k Firestick running 188.8.131.52. with video resolution set to 4k on my Firestick.
The following apps can't play 4k (i.e. I get 1080p):
Netflix - passes Dolby Vision fine but only 1080p video. I have a ticket open with them and they have noted that they are providing the 4k stream to my Firestick. I have the right plan and 150Mbps down so this isn't an issue with my service. They specifically said they are providing the 4k asset to me when they observed it from their end while troubleshooting.
MoviesAnywhere - passes Atmos sound but not Dolby Vision. Application specifically states this device doesn't support 4k playback when a 4k movie is selected. Their tech support claims it's a Firestick issue.
4k channel from Pluto TV - 1080p video only.
In fact the only app that plays 4k properly is Amazon Prime video. Man In The High Castle for example. Oddly, when I play Endgame purchased from Vudu through the Amazon Your Videos section (as the libraries sync now) it plays in 4k (I don't like playing it this way because Amazon still doesn't support Dolby Vision or Atmos when played this way only HDR) but Movies Anywhere with the same movie says the device doesn't support 4k playback.
The fact that no app besides Amazon 4k content plays 4k video tells me their is an issue with the way 4k is implemented in the Firestick OS to 3rd party apps. If it was my TV, a setting, or a speed issue then Amazon content couldn't provide 4k video to me either.
Amazon please debug your 4k implementation with 3rd party providers because right now it's everyone pointing fingers and I've paid good money to ensure I can play 4k content.
I don't doubt your story. I assume you are using a soundbar or receiver? Have you tried plugging the Fire TV 4K directly into the TV? This would eliminate the soundbar/receiver as the source of the problem.
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