Unfortunately, your post has nothing to do with the subject of this thread, which is a problem opening mp4 files. You have a problem saving/exporting such files.
Please do not hijack another user's thread by asking questions which are completely different to what the OP asked, but rather start a new thread with your question, taking care to include full details of your computer (this post will show you what we really need to know), the exact version and variant of Movie Edit Pro which you have and the settings you are using on the mp4 export screen.