As far as we know, you cannot convert RAR archives to MPEG-4 multimedia files (.mp4) and there is also no reason to do so.
The only potential rar to mp4 conversion that would remotely make sense is extraction of multimedia file from RAR archive. If that is the case, just download WinRAR, 7-zip or similar program and explore the contents of the .rar file in question.
If your file is downloaded from torrents or other dubious sources, always make extra effort to verify and check the files for possible virus or malware infections.