Along with revving the iMac line with all Retina displays, Apple has updated iMovie for Mac OS X to version 10.1, adding support for creating, editing, and sharing movies at 4K resolution (3840 x 2160).
You can view pixel-for-pixel 4K video while editing a movie on an iMac with Retina 5K display. (4K export requires a Mac from 2011 or later with at least 4GB of RAM. Full resolution 4K display id supported on an iMac with Retina display or a Mac Pro (2013 or later) when connected to a 4K display.
According to Apple's notes, the upgrade allows you to:
Create and share movies with 1080p HD video at 60 frames per second;
Import movies and trailers from iMovie for iOS (version 2.2 and later), so you can start editing on an iOS device and finish on your Mac.
iMovie 10.1 has a redesigned media view that lets you see more of your library while browsing videos and photos. The Projects view has been tweaked to make it easier to find and open your movies and trailers. Tabs in the browser give you faster access to titles, backgrounds, transitions, and music while editing a movie.
iMovie 10.1 has an option to hide the browser while editing video. There are also 10 addition video filters.
You can obtain the update at the Mac App Store via the Updates option.