Good news: iMovie for Mac support 4K video editing
iMovie for Mac has been updated to version 10 with adding support for 4K video editing and sharing. It also notes that 4K video editing requires a Mac from 2011 or later with at least 4GB of RAM. To playback 4K video with iMovie in full resolution, you'll need an iMac with Retina display or Mac Pro connected to a 4K display.
iMovie's limitation for 4K video editing
Although iMovie for Mac now has support for 4K video editing, but the 4K formats that supported by iMovie is still very limited in MPEG-4, QuickTime MOV, MPEG-2, AVCHD, DV & HDV formats. If you have recorded some 4K video footage with the format of H.265/HEVC, XAVC/XAVC S, Prores, etc which are excluded from iMovie compatible format list, or your Mac is very old and not powerful enough to handle 4K video, you will definitely end up with a failure when perform the 4K video editing process on iMovie.
Best solution to fix iMovie 4K video editing limitation
To achieve a smooth editing workflow of any 4K video format on iMovie, the best solution is to convert and compress 4K video to iMovie preferable video editing file resolution and file format, Here what you need is a iMovie 4K Video Converter.
After many Internet comparison, you may find Pavtube Video Converter for Mac wins a great favor by many users. Different from its competitors, it not only enables you to convert 4K video to iMovie supported editing formats, it also facilitates you to directly output iMovie best editing codec - Apple Intermediate Codec, which is used as a high-quality and less processor intensive intermediate codec for using with HD workflows in iMovie. If you Mac lacks the capability to deal with 4K resolution video, you can also use the application to compress 4K video to 1080p for fluent video editing workflow.
Besides acting as a 4K video converter, the program also performs as a 4K video editor, which helps to offer some basic but beneficial video editing functions. You can trim video length to remove unwanted parts from video, crop video file size, adjust output file aspect ratio, add text/image/video watermark to video, apply special rendering effect, insert external ASS/SSA/SRT subtitles to videos, etc.
How to Convert and Compress 4K Video for iMovie Editing on Mac OS X El Capitan?
Step 1: Add 4K videos.
Open the program, click "File" menu, from its drop-down option, select "Add Video/Audio" to load 4K video to the program.
Step 2: Choose output file format.
From "Format" drop-down option, choose iMovie best video editing codec from "iMovie and Final Cut Ex..." > "Apple Intermediate Codec (AIC)(*.mov) to output.
Step 3: Compress 4K to 1080p.
Open "Profile Settings" window, from video settings panel, click "Size(pix)" drop-down list, choose "1920*1080" as the final output file resolution to downscale 4K video to 1080p.
Step 4: Edit 4K video files.
Select an item to activate the "Edit" menu, on the "Video Editor" Window, you can play and preview the original video and output effects. Also you can flip video, trim, add special effect, reset, and adjust volume.
Step 4: Start the conversion process.
After all the setting is over, go back to the main interface and hit right-bottom "Convert" button to start to convert and compress 4K video to iMovie best compatible 1080p AIC format.
When the conversion is completed, launch iMovie on your Mac computer to import 4K video to iMovie for smooth editing workflow.