Apple TV HD (4th generation) [A1625] should support the latest version tvOS, v18.4.1 at this time.
If the internal updater doesn’t give new updates for you, then please contact Apple for getting later updates.
[Not sure if the version gap between tvOS 16 and tvOS 18 is too big for the internal updater, but in general the version gap can be too big to work automatically (e.g. tvOS 9 to tvOS 18).]
Alternatively, you can try restoring the system software through a computer, but that may not preserve the current content, and you may need to set it all up again. Take good notes of the current apps and accounts needed, before proceeding that way.
Restore your Apple TV through a computer using USB - Apple Support