The easy workaround to get iMovie (10.1.4) work with some MP4 files is to convert MP4 to AIC codec, which iMovie works best with. Here is how.
iMovie only allows you to import the MP4 videos with codec that iMovie can recognize like MPEG-4 and H.264. Since MP4 is a container, not a codec. If the MP4 video is encoded with iMovie unsupported codec, we will fail to import this kind of MP4 into iMovie for editing. Don't worry. To get iMovie work with MP4 files, you'll need to convert MP4 to iMovie compatible formats.
How to Convert MP4 Files to iMovie for Mac OS (Sierra Included)
A professional MP4 Video Converter for Mac - Dimo Video Converter Ultimate for Mac can do the great job even if you are not a pro. With this easy-to-use yet powerful MP4 converter app, you can effortlessly transcode 720p/1080p HD/2160p 4K MP4 files to AIC encoded MOV, which ensures best results when working with iMovie. The Mac video conversion tool can also create files compatible with other non-linear editing systems like Adobe Premiere Pro, Avid Media Composer, FCP, etc, media devices like iPhone, iPad, TV, Xbox One S, PS4 Pro, etc.., or online sites like YouTube, Facebook, Vimeo, etc. Just download the program and follow the step-by-step guide to make your MP4 files compatible with iMovie (10.1.4 included).
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