Google devs confirmed that is true and expected:
https://issuetracker.google.com/issues/119760623
This response is also interesting:
https://issuetracker.google.com/issues/36191392
Chromecast Ultra does support 4K streams, but only with HEVC and VP9 video codecs.
They also suggest querying the playback device for capabilities instead of looking up models. There are TVs and other devices that support the Chromecast platform - the MagentaTV Stick was referenced earlier in this thread.
I made a feature request to add the ability to specify a maximum H.264 Level. This would match most other Plex playback apps.
