m4v?

When you purchase a music video from the iTunes store it comes in the DRM-Protected .m4v format. This is so iTunes can control where you play this purchase, ie on your iPod or your Mac etc.

MPEG-4 is a container format, MP4 files may contain any number of audio, video, and even subtitle streams, making it impossible to guess what type of streams are present in a MP4 file based on its filename extension alone.

Check out this wiki page for FULL detail...
http://en.wikipedia.org/wiki/MPEG-4Part14

The older audio downloads from the iTunes store used to use the .m4p file extension. The "p" showed the file was protected by DRM. Newer downloads for audio files from iTunes Store no longer contain DRM features.
Mac's and PC's that have iTunes installed will open .m4v files in iTunes if the file was double clicked. A Control click (Open With) would allow other apps, like QuickTime Player, open the file.