To my knowledge, there isn't anything RoboHelp may do that would cause an issue.
If you look at the video just using the browser without RoboHelp in the picture, does it look okay there?
How exactly are you presenting the MP4?
MP4 file looks good in browser outside Robohelp. Think the problem is with host site. I download the MP4 file to the host site and use the publish process there. I then place a hyperlink in a Robohelp topic that accesses a URL created by the host site. Think the problem is theirs. Just thought I'd check to see if anyone else has had the problem. Thanks!