My guess is you'd have to buy 4k again if it eventually became available.
Having bought the same movies on VHS/Betamax, later DVD and then Blu Ray or Digital Downloads, it's pretty clear the movie companies really want you to pay and pay and pay....
I've bought movies in SD from iTunes which later became available in HD - different cost, no upgrade pricing which is daft in my view, as I'd probably upgrade some for a nominal charge, but wouldn't buy digitally again at full cost, especially since we now have such a variety of streaming services.
Apple did once offer upgrade pricing for old 128 kbps compressed AAC copy protected music to 256 kbps non copy protected AAC, but I think that's the exception not the rule.
Equally I don't think Apple is really that concerned with quality unless there's a marketing hype that'll sell it, but not only that there'd need to be compelling customer demand and most people will not care.
There really is no reason that Apple could not sell music in uncompressed CD quality or higher, yet they do not as there is little consumer demand.
AppleTV has never really been treated well as a product - AppleTV 4 really should have been 4k capable from the start but it isn't, the next iteration might be if we ever see one, but unless they offer 4k content in iTunes Store I can't see why they'd bother.
It will change, but currently there are far too few titles available in 4k in any format - recently bought a 4k UHD Blu Ray player but only have a handful of discs - suspect the format will die and be overtaken by 4k streaming services from Netflix, Amazon etc...