As far as we know, you cannot directly convert RAR archives to .mp4 files and only "rar to mp4 conversion" that would 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 download from torrents or other suspicious sources, always make extra effort to verify and check the files for possible virus or malware infections.
Open RAR file Open MP4 file