If it doesn't come with 4K touted in the marketing, my best guess is that we'll have to buy an

TV5 to get 4K. It's not that often that Apple builds such a tangible upgrade option into their iDevice products such that just a software update can activate such a feature.
Worse, given the attention this "hobby" gets, if it doesn't arrive with 4K, we might be waiting 3-4 years for a 4K version. In my own experience I was there for the first generation way back in 2007. I piled up a lot of 1080p content for many years (shot with consumer camcorders) awaiting a 1080p

TV that didn't arrive until 2012. Apple gave us Macs capable of handling 1080p content and software able to edit and render it at 1080p. It would even go into iTunes as a 1080p video file just fine and play on those Macs at 1080p. BUT, the one link in the chain-

TV gen 1 & 2- remained capped at 720p for (IMO) much too long.
If this next

TV isn't 4K, I would expect that many will have 4K camcorders/DSLRs and 4K televisions long before Apple gets around to giving us a way to push 4K content we've shot, edited on Macs capable of editing & rendering 4K, to our 4K TVs. If so, they open the door for products like Roku, Amazon, Chromecast and others to step in there first... just as they ceded 1080p playback to products like those for a good while before finally getting around to going 1080p themselves.
I am hopeful they will lead this time instead of following. However, what I think most led to a 1080p

TV was a product problem. Several months before gen 3 arrived, Apple launched iPhones with a touted benefit of being able to shoot 1080p video. How could one play that beautiful 1080p video on their 1080p TVs. NOT through 2nd generation

TV. To solve that problem, they upgraded the "hobby".
The bigger problem in hoping a new

TV is 4K is that the non-"hobby" iDevices like iPhone and iPad are not shooting 4K now. I unfortunately believe we have to see iPhones and maybe iPads shooting 4K video before we get a 4K

TV. So in spite of my own obvious desire for a 4K

TV4, I expect another 1080p

TV with some signature features that can't/won't be implemented on the current 1080p

TV3. But I'd love to be wrong about that... especially with the history of many years between meaningful hardware updates.