M4V is a file container format used by Apple's iTunes application. The M4V file format is a video file format developed by Apple and is very close to MP4 format. The differences are the optional Apple's DRM copyright protection.

To import copy-protected iTunes M4V files to iMovie, your computer must be authorized(using iTunes) with the account that was used to purchase the video. If you want to import M4V(without DRM), you need a software application to convert M4V to iMovie on Mac.

Mac M4V to iMovie Converter help you convert M4V to iMovie workable video formats like MP4, MOV, DV, etc. With this Mac M4V to iMovie Converter, you can import M4V to iMovie for editing. Also, you can convert M4V to other video formats such as FLV, WMV, AVI, MKV etc.

  http://imediaconverter.onapplemac.com/how-to-import-m4v-into-imovie.php