I have five Chromecast Audio devices which produce 3.5mm/optical audio output from one source. I also have a 1st gen Chrome TV with HDMI audio [the "keyhole" shape] which cannot play audio streams shared with Chromecast audio. It will support Spotify, but not as part of the five Chomecast Audio devices in an audio group.
Can a 4th generation Chomecast (product name "Chromecast with Google TV", either HD or 4K) play a Chromecast audio stream shared with Chromecast Audio devices?