You will need certain codec in order fo the media player will be able to recognize the mpeg-4 file. The codec are the ones who translate the video file in order for the media player to recognize it, it the there are certain codecs missing it will just provide an error message that says "unable to recognize file or unsupported video file". The work around to this problem is by playing the video file using another media player such as Quicktime, VLC, etc. But, since you wanted to play the video file on your Windows Media Player, I suggest that you download the Klite Mega Codec Pack. Don't worry it is completely free for downloads no sign ups or purchase.
Now, once you download the Klite Mega Codec Pack follow and read the steps carefully when installing. Now, on the installation there will be an option if you want to associate the codec pack with Windows Media Player, try to mark that so when the program installs all its codecs will be provided to the Windows Media Player so it can recognize video files such as MKV, HD, FLV, MPEG-4, and many more that were not recognize before.
You can download the Klite Mega Codec Pack at the link below:
http://www.free-codecs.com/K_Lite_Me...k_download.htm