You may have noticed Apple mentioned during the last keynote support for 4K editing on the iPad Pro and the iPhone 6s. That app was iMovie and it has now been updated to support the higher 4K resolution.
The new iMovie supports up to 3840x2160 editing on the new iPad Pro, iPhone 6s and the iPhone 6s Plus. On the iPad Pro you can edit three 4K streams at the same time. You can also export videos in 1080p60 now. The 6s and 6s Plus also get 3D Touch shortcut to start video a new film from the homescreen. On the iPad Pro, you get a full height media browser and 1:1 viewer for 1080p videos.
Other features include shortcuts for physical keyboard (such as the iPad Pro Smart Keyboard), option to hide Media Browser while editing, option to disable Ken Burns animation on photo, redesigned Project Details view to watch files before opening them, redesigned Video view, improved inspector on iPad, support for Slide Over and Split View in iOS 9 on the iPad, redesigned Audio browser, performance and stability improvements, and support for additional languages such as Canadian French, Hindi, and Traditional Chinese.
iMovie is a free download and update on iOS.