MPEG-4 Part 14 (.mp4, .m4a, m4p, .m4b, .m4r, and .m4v). Moving Picture Experts Group or MP4 audio. MPEG-4 part 14 or MP4 is an extensive collection of compression schemas for audio and visual digital data. It is most commonly used to store digital video and digital audio streams, especially those defined by MPEG, but can also be used to store other data such as subtitles and still images. The standard is based on the QuickTime file format due to its robust and extensible architecture.

The M4V file format is very similar to the MP4 format but M4V files may optionally be protected by Apple's FairPlay DRM copy protection for the Apple iTunes Store. Since government produced M4V files are in the public domain they should not be DRM protected. They should be playable in the digital library simply by changing the file extension from from ".m4v" to ".mp4" prior to upload.

For more details see the Wikipedia articles: MPEG-4 Part 14
M4V