We do not know about any direct vpj to mp4 conversion at this time. However, it may be possible that you were looking for a way to export VideoPad projects (.vpj) to MP4 or any other multimedia format.
This is doable using VideoPad, but only if you also have access to the original multimedia data referenced in .vpj projects. Without them, it will be impossible to reconstruct the video and export it.
VideoPad is only available for Windows, so MacOS users have to use virtualized Windows environment to install the program. Just make sure you have all the data for .vpj project before you attempt it though.