Sure, you can do that. You can have it render a hyperlink, which will cause the browser to download it and open it with whatever application is registered for that file type. This is similar to how Word documents are rendered (
see Word doc sample).
If you look in gs_BrowserTemplate for the template for Word docs, you'll see this:
<p style="margin-bottom:5em;"><a href="{MediaObjectUrl}" title="Download {TitleNoHtml}">Download {TitleNoHtml}</a></p>
Copy and paste that into the record for the file type you want to update (mp4?).
Roger Martin
Creator and Lead Developer of Gallery Server Pro