The m4v extension indicates a MPEG4 video file.
At the moment there ara many extension used to mark a MPEG-4 video file but the official one is the MP4-M4V.
The m4v extension was introduced and spreaded by Apple Inc. with their products iPod and iTunes in order to recognise what the file is about by the extension.
Before the introduction of this new format, the mp4 file was used to distinguish both audio and video files.
Thanks to the m4v, now, is possible to predict from the extension the nature of the file: a M4v file denote without any doubt, a video file.