A browser does not understand MP4 files the way it does a JPG or the like. You would need to load some HTML code that could play the file. MP4 is also not a format that HTML 4.x understands natively, so your HTML would have to use some media player to actually play the file. Here are some examples:
You would need to craft the appropriate HTML code and, assuming that you're using the WebBrowser control, assign it to the (I believe) DocumentText property. That said, I'm not 100% sure that the WebBrowser control supports HTML 5. If it doesn't, you'd have to use a WebView2 instead.