iMovie for iOS is ready to handle your 4K video editing
 

If you're determined to produce a 4K masterpiece on an iPhone 6s, 6s Plus or iPad Pro, you now have one of the tools you need to get the job done. Apple has delivered an iMovie update that switches on 4K editing for its most powerful iPhones and iP...

http://www.engadget.com/2015/09/16/imovie-4k-update/