It seems that arf to mp4 conversion mainly exports WebEx web conferencing video recordings (.arf) to MPEG-4 video (.mp4) files.
The most convenient way to achieve this conversion is to download the latest version of Cisco WebEx, a program that allows free export to several multimedia formats, including WMV, SWF, and possibly MP4. The program is available for both Windows and MacOS.
You can also use WebEx Recording Editor to edit the .arf recording before converting it to another format.
If exporting to MP4 format does not work for you, you can always save it as WMV and use any of the available WMV to MP4 converters to get your MP4 file.