There are two definitions for the term M4V. The first is that raw MPEG-4 Visual bitstreams are named .m4v.
The second, and much more likely, is that you have legally downloaded a video file from the Apple iTunes store and it has the M4V extension. These files can be movies, TV shows or music videos and all will include Apple's FairPlay DRM copyright protection.
Tp play these files, iTunes must authorize the video. If your file is unprotected however, you can change the file extension from ".m4v" to ".mp4" or play with the following list of software (also linked below):
Apple iTunes, QuickTime Player, RealNetworks RealPlayer, Media Player Classic and Miro. WMP cannot play these files.