I've seen a couple of postings on this subject but they talk about converting the mp4 to another format. Is there a program that can add/remove subtitle tracks in an mp4 file?
Adding Subtitles to MP4: Download MP4Box and Yamb, and extract them from their zip files onto your PC. Run Yamb, Double click on the "Advanced Settings for Yamb" entry. Click on the Add button and locate the MP4 file that you wish to subtitle. Add the file to the MP4 File Creation window. Click on the Add button again, and locate the SRT subtitle file that will be added to the video. Change the output file's name to make a separate MP4 file, unless you wish to overwrite the original MP4 video. Click on Next, and wait for Yamb to process the file. The time required will vary based on the video's length and file size. When the file finishes, click "Finish" to close Yamb, or click "Back" if you have any other videos to add subtitles to.