What is the difference between an .MP4 and .MPG?

Can I play an .MP4 in windows media player?http://www.fileinfo.net/filetypes/video
.mp4 - MPEG-4 Video File - The MPEG-4 video format uses separate compression for audio and video tracks; video is compressed with MPEG-4 video encoding; audio is compressed using AAC compression
.mpg - MPEG Video File - COMMON video format standardized by the Moving Picture Experts Group (MPEG); typically incorporates MPEG-1 or MPEG-2 audio and video compression; OFTEN used for creating downloadable movies.

http://support.microsoft.com/kb/316992
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:
http://www.ligos.com (http://www.ligos.com)
For more information about EnvivioTV , visit the following Envivio Web site:
http://www.envivio.com/products/

Use VLC Player to play .mp4 files.