Download a program that already plays back MP4 files. You can download a free VLC player or Video Dub player, both of which will allow you to open MP4 files. However, this option does make you have to add another program to your computer, which is counterproductive if you are trying to save space. You can simply upgrade your existing media player.
Double click on the desktop icon for "Windows Media Player." If you do not already have Windows Media Player saved as an icon to your desktop, you can open it by clicking on "Start" and scrolling through "All Programs."
Click on the pull-down menu for "File" located at the top of the player in the toolbar. From the list of options, select "Open."
Search your hard drive for the MP4 file you wish to open. Your personal files will be saved in one of the folders in the "C" drive of "My Computer." Double click the file to open it into Windows Media Player.
Identify whether the file was allowed to open successfully. You may see a message box that pops up saying that it was unable to open the file. If this happens, click on the option to download the appropriate codec to allow the file to open. You do not need to know any specific codec names to download; it will only give you an option to press "OK" if you wish to download.