Popularized as the default file-type in iTunes, MPEG-4 audio files are increasingly common on today's Internet—but haven't yet managed to replace MP3 files (a format which is based on MPEG-1.) As such, many music players on the market today still don't support MPEG-4 files. If you have such a device, you'll need to convert your MPEG-4 files to play them on your device. Happily, every major platform has free tools available for the job.
The MPEG-4 standard is a container format, meaning an MPEG-4 file could conceivably be a video file or an audio file. MP3s being are an audio-only format, and you cannot convert a MPEG-4 video file to MP3 without losing the video completely. Be sure the MPEG-4 files being dealt with contain only audio.
If you own a PC running a version of Microsoft's operating system you can use dbPowerAmp to convert your MPEG-4 files to MP3s. Install the program, as well as the MPEG-4 codec for the program. Once both are installed, using the program to convert a given file is as easy as right-clicking the file in question, then clicking the dbPowerAmp option in the resulting menu.
Mac and Windows: iTunes
Windows and Mac users alike can use iTunes to convert MPEG-4 files to MP3 quickly and easily. Open iTunes, then click click "iTunes" on a Mac or "Edit" in Windows, followed by "Preferences." Click the "General" tab, then the "Importing Settings" button. Select “MP3” as the format to which you'll import. Now, within iTunes, find the MPEG-4 files you wish to convert, then click “Advanced,” then click “Create MP3 version.” MP3 versions of the files will be imported to your iTunes library.
Linux users can use SoundConverter to make MP3 files out of MPEG-4 audio files. This straight-forward program allows you to convert any music file to any format you wish. Installing the program is simple: just find the “soundconverter” package in your distribution's package manager and install it. Start the program, then click “Edit” followed by “Preferences” and change the export format to “MP3.” Then click “Add file” to add individual files or “Add folder” to add an entire folder worth of MPEG-4 files. When all files are added click “Convert” and MP3 files will be created using the MPEG-4 files.