The easiest way to do that is to build a HTML page that plays that video, then in Board use the "Viewer" object (which is a browser) and type the URL of that page.
For example, I build a page YourVideoPage.html which contains a video element to play your MP4 video, something like this:
<video width="320" height="240" controls="controls">
<source src="yourmovie.mp4" type="video/mp4" />
<source src="yourmovie.ogg" type="video/ogg" />
Your browser does not support the video tag.
</video>
Then in BOARD, use the Viewer
object on the screen and configure it with the URL of your page:
http://..yourserver/YourVideoPage.html
This should work fine.
If the video is on YouTube, on Vimeo or similar, simply go to the video then copy the URL from your browser and paste into the Viewer
object's URL property in BOARD.