I just started Apple TV on my Ultra 4850, which has a WiFi connection (I'm not using Ethernet since WiFi is over twice as fast). I started playing a 4K video in my library (Star Wars: A New Hope). I'm not seeing any problems playing the video. This specific movie is 4K/Dolby Vision, so it should be maxing out the Apple TV app. The hardware in the Roku Soundbar should have no issues with playing any such content, assuming you aren't having any network issues. Perhaps Apple might be having issues at the server end. But as I said, it seems to be doing just fine right now.
Dan
Roku Community Streaming Expert
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