I just don’t think AirPlay has been redesigned for 4K.
I was always under the impression that it sends incoming compatible (AVC, these days also HEVC) stream over the net just verbatim.
Does it really re-encode it on the fly? Why?
PS iPhone havs a on-board hardware encoder, but my 2013 MacBook Pro definitely doesn't. Still, I can easily send my 4K home video from iTunes over AirPlay. Even with HDR10. Even with E-AC-3 Atmos. And subtitles.
I bet it would do also Dolby Vision, iTunes also happily ingests such a file, but aTV Computers app resolutely excludes them from the Library listings
![Frown :( :(](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
So I reckon, it just sends the file contents (streams) unaltered.
PS Sending a iPhone camera clip might be another story, as it has quite high bitrate (close to 60MBps from my iP8). Maybe the Photos app takes it down to lesser bitrate or even lesser resolution. 60MBps is way too much for the aTV 4K native decoder. Infuse can handle it.
PPS Watching a stream from iCloud Library definitely is 1080p and comes as such down from the cloud already.