https://www.apple.com/uk/apple-tv-4k/specs/
Now supporting HDR and Dolby Vision, along with Apple Airplay 2 speakers "coming soon."
Press release: https://www.apple.com/newsroom/2017/09/apple-tv-4k-brings-home-the-magic-of-cinema-with-4k-and-hdr/
HE-AAC (V1), AAC (up to 320 Kbps), protected AAC (from iTunes Store), MP3 (up to 320 Kbps), MP3 VBR, Apple Lossless, FLAC, AIFF and WAV; AC‑3 (Dolby Digital 5.1) and E‑AC‑3 (Dolby Digital Plus 7.1 surround sound)