Adobe Premiere Pro projects (.prproj files) can be exported to many video formats, including MPEG-4 (.mp4). So that is your best, and most likely also only 100% reliable way how to export or convert prproj to mp4 format.
Just keep in mind that these projects do not contain multimedia audio of its own, just settings and references that are used to generate the output. So without the relevant multimedia data, it is impossible to perform prproj to mp4 conversion.
Updated: July 3, 2023