ARF to MP4 conversion seems to mainly represent export of 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 format. The program is available for both Windows and MacOS.
You can also use WebEx Recording Editor to edit the .arf recording before you convert it to some other format.
If export to MP4 format does not work for you for reason, you can always save it as WMV and use any of the available wmv to mp4 converters to get your MP4 file.