The only mp4 to vpj conversion that would make sense seems to be import of .mp4 files (MPEG-4) to VideoPad and using them for making a new project (saved as .vpj). But this appears to be more of a workflow than actual conversion.
Convert MPEG-4 video file format to VideoPad video editor project.
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert mp4 to vpj on Windows
A video editing software for Windows
Convert from mp4 file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.