The following information is taken from Tim Fisher’s guide on About.com : http://pcsupport.about.com/od/fileextensions/f/mp4file.htm

MP4 files can be opened with Windows Media Player, and other multi-format media players like VLCQuickTime, and more.

Important: If you find that your favorite video player doesn’t open MP4 files, you may need to install an MPEG-4 codec. An MPEG-4 codec is a small piece of software that allows your computer to recognize MP4 files and have them play properly in whatever player you use.

I highly recommend XP Codec Pack, a completely free collection of popular codecs that works inWindows 8Windows 7Windows Vista, and Windows XP. After installation, you’ll be able to play MP4, as well as almost all other popular video formats, in your favorite player.

More info found on Google

What is an mp4 file?
MP4 files (MPEG-4 Part 14) are multimedia files. MP4 is a container format that can store video, audio and subtitle data (as well as more depending on content). Since stores like iTunes uses this container format, and it is used with iPod and PlayStation Portable (PSP), MP4 files have become more common.

How to play MP4 files – AfterDawn: Guides
www.afterdawn.com/guides/archive/how_to_play_mp4_files.cfm

What plays mp4 files?
A few samples of free MP4 players include QuickTime and iTunes (yes, iTunes is a type of player, using both the .m4a file format for music and .mp4 for movies) by Apple, GOM Player, VLC, Windows Media Player, Media Go by Sony and of course RealPlayer.

MP4 Players – All About MP4s — RealPlayer
www.real.com/resources/mp4-players/

What is a codec pack?
The Media Player Codec Pack supports almost every compression and file type used by modern video and audio files. The package is simple to install, while also offering advanced settings to the high end user: For simple installation select “Easy Installation”. Compression types that you will be able to decode include:

Media Player Codec Pack for Microsoft Windows
https://www.mediaplayercodecpack.com/