As far as we know, vpj to mp4 conversion is related to export of VideoPad projects (.vpj files) to MPEG-4 format, mostly for the purpose of video upload to YouTube. You can do this only in VideoPad by using the "Export Video" button in the Home tab toolbar and selecting the MPEG-4 format (.mp4 files) as output before saving it.
Just keep in mind that you can potentially export .vpj files to video only if you have access to the original multimedia data which are referenced within the .vpj projects. Without the original data it's impossible to export the video and the lone .vpj file is more or less useless.Open VPJ file Open MP4 file
A video editing software for Windows