The Apple TV is a product that has never really been a core focus for Apple, and it has traditionally lagged behind its competitors. For example, up until September, 2015 when the fourth-generation model was introduced, it didn't even have an App Store, so you only got the pre-installed apps.
That model, which only supports resolutions of up to 1080p, still hasn't been refreshed. Starting at $149, it's among the most expensive set-top boxes that still don't support 4K UHD. All of its major competitors do, such as Amazon's Fire TV, Microsoft's Xbox One S, the Roku Ultra, and even Google's Chromecast Ultra.
But according to a report from Bloomberg that should surprise exactly no one, the fifth-generation model will support 4K, as well as "more vivid colors", which likely means that high dynamic range (HDR) support will be included as well. Indeed, users will finally be able to AirPlay their 4K videos that they recorded on their iPhones a year and a half ago on their Apple TVs.
If you're hoping for a firmware update to the current model, that's not going to happen for a couple of reasons. Support for 4K would have required a better chip than the A8 that was used, and it also has the HDMI 1.4 spec, which will output 4K, but only at 30fps. It's likely that the next model will use HDMI 2.0, or if Apple really wants to stay ahead of the curve, HDMI 2.1.