How to convert .m4r audio to .mp4 format.
Converting iPhone Ringtone Files directly to MPEG-4 Videos (.m4r to .mp4 conversion) is certainly possible, but actually not necessary. All you need to do is simply rename the file's suffix from M4R to MP4.
Alternatively, you can use an audio converter, such as the Tiny Audio Converter.
An iPhone ringtone creator
Audio converter for Mac