The leaked HomePod firmware remains treasure trove of potential future Apple device features-- the latest being developer Guilhereme Rambo's discovery of a reference to a 4K HDR display mode.
The HomePod lacks a display, and the only Apple devices with native 4K capability are the latest iMac models, suggesting the disovery pertains to the Apple TV. In addition, Rambo discovered strings kCADisplayModeDolby and kCADisplayModeHDR10, suggesting a future Apple TV might even include support for the Dolby Vision and HDR10 colour formats.
Lending further credibility to such rumours is the fact Apple is listing select movies as 4K and HDR in the iTunes purchase history. This was discovered by MacRumors forum member Tomas Jackson, who shared a screenshot of his purchase history of the 2016 film Passengers. It lists as "Movie (4K, HDR)." However, as it currently stands the film can only be played in 720p and 1080p resolution, with no HDR.