Windows Media Player can't play .mp4 file format directly.

MPEG-4 is an International Standards Organization (ISO) specification that covers many aspects of multimedia presentation including compression, authoring and delivery. Although video compression and file container definition are two separate and independent entities of the MPEG-4 specification, many people incorrectly believe that the two are interchangeable. You can implement only portions of the MPEG-4 specification and remain compliant with the standard.

The MPEG-4 file format, as defined by the MPEG-4 specification, contains MPEG-4 encoded video and Advanced Audio Coding (AAC)-encoded audio content. It typically uses the .mp4 extension. Windows Media Player does not support the playback of the .mp4 file format.

Windows Media Player can play .mp4 file format with installing mp4 codec.

You can play back .mp4 media files in Windows Media Player when you install DirectShow-compatible MPEG-4 decoder packs. DirectShow-compatible MPEG-4 decoder packs include the Ligos LSX-MPEG Player and the EnvivioTV

EnvivioTV

MPEG-4 Player for Set-Top Boxes, PCs, and Mobile Devices

EnvivioTV is a software video and multimedia player that allows a user to view any ISO MPEG-4 compliant content. The software allows the user to interact directly with broadcast media, experiencing MPEG-4 interactive capabilities while selecting and interfacing with multimedia objects.

The stand-alone version of the player can be integrated or ported onto any media device, including set-top boxes, PCs, PDAs, and game consoles.

The plug-in version supports QuickTime, RealOne, Real Player, and Windows Media player products. EnvivioTV was certified by RealNetworks quality labs and is part of RealNetworks auto-update program as the reference MPEG-4 plug-in within RealNetworks player (8.0 plus and beyond).

Official website: http://www.envivio.com/

Microsoft has chosen to implement the video compression portion of the MPEG-4 standard. Microsoft has currently produced the following MPEG-4-based video codecs:
- Microsoft MPEG-4 v1
- Microsoft MPEG-4 v2
- Microsoft MPEG-4 v3
- ISO MPEG-4 v1

MPEG-4 video content can be encoded and stored in an .asf file container by using Windows Media Tools and Windows Media Encoder. You can then play these files in Windows Media Player.