Confirming earlier rumors, this box has an HDMI 2.1 video output, however as 9to5Mac notes, despite code leaks that mention support for a 120Hz mode, at launch the device's "high frame rate HDR" will top out at 60 FPS. That could change with the next version of tvOS — or once 4K TVs aren't as finicky with their support — but it's something to keep an eye on.
One other thing to be aware of, as Dan Moren pointed out on Twitter, is that this is the first Apple TV box with Thread built-in. Thread is a low power mesh networking technology for home automation devices with backing from a number of big name companies, and they plan to use it to support Project CHIP (Connected Home over IP) which is an interoperability standard for smart homes.
Thread support is limited to a few devices so far, but it could make this box a lot more useful as a hub for your future smart home. The HomePod mini also includes Thread technology, however the now-discontinued original HomePod did not, which may be a part of the reason why it got the axe and could feature in upcoming devices.