I wouldn't worry about iTunes lacking "4K" content yet, as virtually all movies are made in 2K (2048x1080 - so only slightly higher than 1080p) anyway and then upscaled. Older movies shot in the pre-digital era can of course be re-scanned in 4K, but if you're looking at a film made within the last 20 years or so, chances are it will be in 2K.
There are exceptions naturally, once in a while a director will prioritise image quality and insist on a 4K digital intermediate, but you can pretty much count the number of movies with those per year on the fingers of one hand.
You can check what resolution a film's DI is on IMDB in the More/Technical Specs section. In the case of Valerian you'll see this -
Runtime 2 hr 17 min (137 min) (USA)
Sound Mix
DTS (DTS: X) |
Dolby Surround 7.1 |
Dolby Atmos |
Dolby Digital |
SDDS
Color
Color
Aspect Ratio 2.35 : 1 (anamorphic)
Camera Arri Alexa XT Plus, Leica Summilux-C, Summicron-C, Fujinon Alura and Angenieux Optimo Lenses
Negative Format Codex
Cinematographic Process ARRIRAW (3.4K) (2.8K) (source format)
Digital Intermediate (2K) (master format)
Dolby Vision
EclairColor
Printed Film Format D-Cinema (also 3-D version)
DCP
That said, iTunes 4K releases (as well of course, UHD discs) will give you HDR and a higher bit-rate, which can make things look nicer despite the lack of a resolution increase.