GoToWebinar video-conferencing app allows users to export their session files (.webinar) to MPEG-4 video file format (.mp4), which is what webinar to mp4 conversion more or less represents. It does not seem to be possible to achieve this using any 3rd party app, so using the original app looks like the only way to convert webinar to mp4.
Open WEBINAR file Open MP4 fileA video conferencing software from Citrix