There would seem to be no reason Apple couldn't significantly improve the OS and UX with the existing hardware, and not require a new ATV 5 to do so. If Apple comes out with their streaming TV service, I would expect that to go hand-in-hand with a major OS upgrade on the ATV 4.
None of these things were possible with the old ATV due to multiple hardware limitations.
I realize my post wasn't clear. When I said a more fully featured ATV 5, I meant 4K and HEVC support (at the very least), and the release would coincide with the streaming TV service. I think the ATV 4 will support the streaming TV service, because why wouldn't Apple want the subscriptions, but I don't see the ATV 4 being the latest gen ATV for as long as the ATV 3 was.