An M4R file contains a custom iPhone ringtone created with Apple iTunes, a media player bundled with Apple devices. It can only be created from a song purchased from the iTunes music store that allows ringtone creation. M4R files are automatically transferred to an iPhone when the phone is synced with the computer using Apple iTunes.

You most likely will only encounter an M4R file if you are transferring a ringtone from a computer to an iPhone. You can choose to sync the iTunes library on your computer with your iPhone, which automatically transfers M4R files from your iTunes library to your iPhone. You can also manually transfer M4R files on your computer to your connected iPhone with a third-party file manager. After transferring an M4R file to your phone, you can set it as the ringtone on your iPhone by selecting Settings → Sounds & Haptics → Ringtone. M4R files are typically created with iTunes but may also be created with third-party software programs or downloaded from the Internet. M4R files not created with iTunes most likely do not include copyright protection. These files may be played by other programs such as VLC media player.