First things first, you need to set up observers for the errors coming off the video node in your app, then check the telnet log. UHD/4K seems like it would be a problem, at least during troubleshooting. We use HLS 24/7, so it definitely works. I would check to see if Wistia is using some kind of tokenization on the manifests and/or segments.
Wistia handles all of the details of adaptive streaming automatically - and states only that it outputs in an HLS format. Since they are 3rd party, I will not have access to their server logs - only their analytics - which shows now play activity on the test videos.
In my previous request for assistance form Wistia they stated only that Roku is not a "supported" platform - but that I may be able to make it work - but I'm on my own for figuring it out.
I will see how they respond to if they are "using some kind of tokenization on the manifests and/or segments"