What Internet connection speed do you need to watch iTunes's 4K movies?
Apple recommends a minimum speed of 25 Mbps for 4K streaming


Find and watch movies with 4K, HDR, or Dolby Vision - Apple Support

A nice tool to estimate what bandwidth is needed for various video formats is the Black Magic Disk Speed Test app: https://itunes.apple.com/us/app/blackmagic-disk-speed-test/id425264550?mt=12