Definition - What does M4V mean?
An M4V is Apple’s video container format that is very similar to mp4, but differs in terms of protection and copyright. M4V is essentially an MPEG4, and can be converted if the file is not protected by iTunes. M4V format files can be played using common video media players such as VLC Media Player, RealPlayer, QuickTime and iTunes.
Techopedia explains M4V
M4V is the encoding technique used by Apple to encode movies, music videos and TV episodes on the iTunes Store. Apple is strictly concerned about the copy protection and rights of features used by consumers, so the M4V format is an attempt to protect the videos on iTunes. Only the computers authorized by an iTunes account that are used in purchasing the videos are able to play them if the M4V is protected by Apple’s FairPlay DRM copy protection. The format may be possible to convert using QuickTime Media Converter or other similar software.