How to upload the 4k videos taken to YouTube?
This is a limitation of Apple's Photos.app as of iOS 10.3.1.
Photos.app limits video uploads to 720p/1080p.
To upload 4K videos from your iPhone to YouTube you can use Apple's iMovie for iOS. iMovie for iOS is free for all users since April 18 2017.
Unfortunately you have to use a second app to accomplish a 4K upload.