First, how did you find out movies are not playing on your iPhone in 4K resolution?
What is the display resolution of your device for it to play in 4K?

Phones that provide display resolution 3840 X 2160 (4K) 2960 X 1440 (Quad HD+) 2560 X 1440 (Quad HD). That means you can watch high-quality videos on these phones with fast processing speed.
iPhone X & XS has --> 2436 x 1125-pixel resolution and this is not a 4K display,
iPhone 12 Pro has ---> 2532 x 1170-pixel resolution and hence is not a 4K
iPhone 13 ---> 2532x1170-pixel resolution at 460 ppi and hence is not a 4K
iPhone 13 Mini ---> 2340x1080-pixel resolution at 476 ppi and hence is not a 4K
While these iPhones can record 4K video.
My information says that 4K resolutions display is a waste on mobile devices as you (a normal human eye) may not be able to notice the difference on such small screens.
