According to Apple support, 4K video cannot be downloaded for offline viewing, regardless of your device - ‘You can download a copy of an HD movie to your iPhone, iPad, iPod touch, Mac, or PC, but you can't download a 4K version’.
Quite a shame since the 16” MacBook Pro sports a quality 3K [3072x1920] display and the 12.9” iPad Pro is not far behind with 2.7K [2732x2048]. Hell, even the 6.5” iPhone 11 Pro Max outpaces 1080p with a solid 2.6K [2688x1242].