Users looking for mepj to mp4 conversion are actually looking for a way to export Movavi Video Editor projects (.mepj) as MP4 video, typically when they got the .mepj file from someone instead the actual video.
While it is possible to open and export the video using the VE, it will only work if you also have access to all the multimedia referenced (linked) to the project. Otherwise it's just bunch of settings in XML format, not really anything you can export.
And even if you got the multimedia data, without the commercial version (paid license) the result will leave a watermark on the video. So this means, it's is probably best to ask for the export from the person that send you the .mepj file in the first place.
Updated: July 21, 2023