An upcoming home cinema feature will soon enable Apple HomePod to blast Dolby Atmos audio from Apple TV 4K. The feature, which is on the way as part of a future firmware update, will also support 5.1 and 7.1 channel audio (via The Verge). Only the regular HomePod will get the option, however. The new HomePod mini won’t support Atmos.
The home cinema mode will work if you connect a single HomePod to your Apple TV 4K, but you'll likely get a better Atmos spacial audio effect if you can link two HomePod speakers to the streaming device. In any case, a HomePod setup will probably sound better than most built-in TV speakers.