Open Windows Media Player on your computer by click the "Start" button and "All Programs" and "Windows Media Player."
Click the "Play" menu button at the top right corner of the Windows Media Player main window. Click the "Lyrics, Captions, and Subtitles" option.
Navigate to where your MP4 video file is in the explorer window of Windows Media Player. Double-click the MP4 video file, and it will begin to play with the subtitles enabled.