For Apple iTunes and iPod/iPhone/iPad.

Input: M4V iPhone (IOS) file can be created only when the MP4 file is provided

More on M4V iOS (iPhone) file

M4v is used in Apple iTunes and iPod. It can be renamed to MP4 file extension.
It allows you to combine different multimedia streams (audio and video) into one single file.
It also allows combining Caption or Subtitles together with the audio and the video.
Apple QuickTime Player also supports the M4V format and has its own UI to browse between languages.