ok, I stand corrected. It's hanky, but it works..
If you already have the entitlement, you CAN buy it again ONLY from iTunes on PC/Mac.
So I had Sherlock Holmes. It was upgraded to HD as a result of MA (and thus "4K" showed up). It was originally an SD digital copy. I played it and like everyone is saying, it was 1080p Dolby Vision. I went into iTunes Store on my Mac and chose "send as gift" and bought it, filled in the email address and it generated a code. I redeemed that code and it now plays in 4K Dolby on my Apple TV.
So I can live with this. Basically studios aren't going to give you free 4K versions on iTunes from this. It is obviously not awesome, but I get it...
tl;dr it looks like you won't get iTunes 4K copies out of this. You WILL get HD HDR copies.. but actual 4K copies you will have to buy on iTunes. The only unknown is if a 4K purchase on Vudu will transfer to iTunes. a 4K purchase on iTunes WILL NOT transfer to Vudu as 4K, only HDX (personally verified)
tl;dr 2 if you get "screwed out of" a 4K iTunes copy from this, you have to load up iTunes on a PC or Mac, and choose "send gift" of the HD/4K copy to yourself. Then redeem that code and you will have the 4K version.