Convert PART to MP4

Converting Partially Downloaded Files directly to MPEG-4 Videos (.part to .mp4 conversion) is not possible.

You can try renaming the file's suffix from PART to MP4, and see whether the file is playable via any media player. In most cases, however, the file will be corrupted.

Your only option is to download the file again. Ideally, the system will allow you to resume downloading the already partially downloaded PART file.

In a word, direct part to mp4 conversion is not possible.
So don't expect any so-called part to mp4 converter or part to mp4 free online conversion tool available.