4K content is already there, if youre on an updated device and browse the movies itll say 4K and an HDR format instead of HD if its the newer version of the movie. I own John Wick in 4K right now, and on the front page theres Wonder Woman, Baby Driver, and probably some others.
Redeemed codes get updated as well. So any code that would normally redeem in HD will redeem automatically in 4K if thats on the store right now, and if it redeems in HD and it gets upgraded later your version becomes 4K for free automatically