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.
Another reader points out the 2016 film Fantastic Beasts and Where to Find Them is also listed as 4K and HDR, a fact confirmed by MacRumors. One also has to keep in mind the latest iPhone models (since the iPhone 6s) can shoot 4K video, but the Apple TV can only play such video at reduced 1080p resolution.
So, will Apple actually release a new, 4K-capable Apple TV? Perhaps. In another deep dive into the HomePod firmware, MacRumors discovered the string J105a-- a possible reference to the 5th generation Apple TV, whose internal codename according to a February 2017 Bloomberg is... "J105."
Go Apple Listing Select Movies as 4K and HDR in iTunes Purchase History (MacRumors)
Go Apple Vowed to Revolutionise TV. An Inside Look at Why it Hasn't (Bloomberg)