http://www.bigasoft.com/articles/m4v-to-itunes-solution.html
Though M4V is developed by Apple and is used to encode TV episodes, movies, and music videos in the iTunes Store, not all M4V files are produced by Apple and supported by Apple devices like iTunes, Apple TV, iPad. So if you got the M4V video from someone or somewhere instead of iTunes Store, iTunes will refuse to import and play those M4V files (especially the M4V movies that are not purchased through iTunes or the M4V video created using CloneDVD Mobile, downloaded from websites, converted by HandBrake or Toast 10 and etc).
In order to successfully import M4V to iTunes to play M4V in iTunes or sync M4V to iPad/iPhone/iPod/Apple TV, a recommended solution is to convert M4V to a format that iTunes can handle such as MP4, MOV.
Bigasoft iTunes Video Converter is just the right M4V to iTunes conversion software! It can fast convert iTunes unsupported M4V video to iTunes supported one. Besides converting M4V to iTunes, Bigasoft iTunes Video Converter also helps to convert other video formats like AVI, WMV, MKV, WTV, VOB, FLV, MPEG, 3GP, MXF, MP4, MOV, MVI, RMVB, F4V, SWF, ASF and etc to iTunes MP4 or MOV.
The following is a step-by-step guide on how to convert M4V to iTunes with the help of Bigasoft iTunes Video Converter. Note: This guide does not apply to convert a protected M4V by using Apple's FairPlay copyright protection.