You can download Quicktime 6.0 or later and after installing it will be able to play the file

get vlc :

www.videolan.org/vlc/

That will play it for you, it is open source and free and it will play just about anything :)

Other then that you will most likely need to get the relevant codecs :)

You can use gspot to figure out which ones you require :)

http://www.headbands.com/gspot/download.html

If you need help finding any codecs please post back :) There are also codec packs but it is not recomended you install too many as it will clog things up and slow things down.

Thanks and kind regards

Gecko

Used gspot could not figure it out.  Is Windows Media Player not supposed to play mp4?

ok open gspot up and there is a file menu --> go to open --> find your mp4 file, click on it, click ok then it will load the mp4 file into gspot.

You can also find the ..... button near the top left of the program and that will do the same thing.

Here is gspots mini faq :

http://www.headbands.com/gspot/minifaq.html

ya i tried that i get not a valid avi file.

In wmp help it shows this as compatable file types that it supports :

==============================================

File types
The File Types tab displays the following available file types (formats) that Windows Media Player can play. Select the check box next to a file type to make Windows Media Player the default player for that file type. Clear the check box next to a file type if you want to make another player the default player for that file type.

If a selected check box is dimmed, Windows Media Player has only partial ownership of the file type. Multiple file name extensions are assigned to the file type, but the Player only plays some of those extensions by default. To give the Player full ownership of a file type, double-click the dimmed check box.

File type Description
Windows Media Audio file (wma) Windows Media files with .wma and .wax extensions.
Windows Media Video file (wmv) Windows Media files with .wmv and .wvx extensions.
Windows Media file (asf) Windows Media files with .asf, .asx, .wpl, .wm, .wmx, .wmd, and .wmz extensions.
Microsoft Recorded TV Show Microsoft Recorded TV Show files with .dvr-ms extensions.
DVD Video DVD video files with .vob extensions.
Music CD Playback Audio CDs and files with .cda extensions.
MP3 audio file (mp3) Files with .mp3 and .m3u extensions.
Windows video file (avi) Files with .avi extensions.
Windows audio file (wav) Files with .wav extensions.
Movie file (mpeg) Files with .mpeg, .mpg, .mpe, .m1v, .mp2, .mpv2, .mp2v*, and .mpa extensions.
MIDI file (midi) Files with .mid, .midi, and .rmi extensions.
AIFF audio file (aiff) Files with .aif, .aifc, and .aiff extensions.
AU audio file (au) Files with .au and .snd extensions.



*To play .mp2v files, you must have a software or hardware DVD decoder installed on your computer.

 Notes

Other multimedia playback programs on your computer may interfere with your Player file types settings or prevent you from specifying Windows Media Player as the default player for a selected file type. To override the file types settings and specify Windows Media Player as the default player for all of the file types in the list, use the Set Program Access and Defaults feature in Windows (click Start, point to All Programs, click Set Program Access and Defaults, and then choose the Custom configuration that specifies Windows Media Player as the default media player).
Clearing the check box next to a file type restores the file type association to the previous default player, if one was specified. If no default player was previously specifed and you try to play a file, a Windows dialog box will be displayed so that you can select the appropriate program to play the file.
Your AutoPlay settings may affect your Player file types settings. For more information about AutoPlay, see Windows Help.
The Player can play additional file types but cannot be set as the default player for them. For more information about all of the file types that Windows Media Player supports, see the Microsoft Web page.

===========================================

Other then that using quicktime as per messhalmans suggestion or vlc as per my above suggestion should work.

There is an alternative players to quicktime like quicktime alternative :

http://www.free-codecs.com/download/QuickTime_Alternative.htm

to read more and how to play the MP4 files see :
http://forum.doom9.org/showthread.php?s=&threadid=62723
and this explanation on how to play MP4 files :
http://www.1src.com/forums/printthread.php?t=69398

try ffdshow

http://sourceforge.net/projects/ffdshow

I use this on an htpc to play mp4 stuff
 

or see here

http://codecs.necromancers.ru/

Winamp is one of the best media players around and will play mp4 without needing any plugins.
www.winamp.com

so is videolan