Apple released iMovie 10.1 for the Mac on Tuesday with support for 4K and 1080p slow motion video. The update coincided with today's Retina iMac updates.
iMovie for the Mac gets 4K video editing support
Along with 4K video support, the 10.1 update includes 60 fps slow motion video support, the ability to import movies and trailers from iMovie 2.2 and later on iOS, an improved media viewer, and a new projects view tabs for quicker access to titles, transitions, backgrounds, and music.
4K video editing requires at least a 2011 Mac with 4 GB of RAM, and full resolution playback requires a Retina, or a Mac Pro with a 4K display.
The iMovie 10.1 update is free and available via Apple's Mac App Store app.