Many people watch films and series only online, but the ability to download them to watch later without an Internet connection is also important. Sometimes the Internet just doesn't work, and you need to keep yourself busy.
If you're hoping to be able to watch iTunes movies in 4K without an internet connection, you should be disappointed – you can't do that yet. 4K video can only be viewed online. You can still download movies and TV shows, but only in HD quality.
Some movies can be downloaded in HDR10 or Dolby Vision, but these are still HD quality options.
Apple has not yet explained the reason. Some believe this is due to the large size of 4K movies and the fact that Apple TV is low on memory. Licensing issues may also be the cause, caused by Apple selling 4K movies at the same price as HD.
In any case, this remains a problem for now. Perhaps it will be fixed in the future, but for now, you will not be able to download movies in 4K via iTunes.