7z to mp4 conversion is actually not a file conversion, certainly not video conversion as the MPEG-4 output format would imply. It's merely the process of extracting the MP4 file from 7-zip archive.
Users that are searching for ways how to convert 7z to mp4 are usually not familiar with 7-zip and often download .7z files from Internet thinking that these are videos. All you have to do is to install and use the free 7-zip and extract the files.
As always, you should be extra careful when downloading archives from Internet, especially from not reliable sources. It's easy to infect compressed archives with malware, ransomware or other malicious code.