What Software Do I Need To Watch A Mp4?

Hi

From the Microsoft site:


  Windows Media Player does not support the playback of the .mp4 file format. 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.

For
  more information about the Ligos LSX-MPEG Player, visit the following Ligos Web
  site:For more information about EnvivioTV , visit the following Envivio
  Web site: 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

I hope this helps
Marcelle