MP4 video is not playing - IIS/Plesk

You have to add the MIME Type to resolve this issue. You can add the MIME type in Virtual Directories > MIME Types or in web.config file. 
For Web.Config file you need to add below section in configuration.
<?xml version="1.0" encoding="UTF-8"?>
<configuration>
  <system.webServer>
    <staticContent>
        <mimeMap fileExtension=".mp4" mimeType="video/mp4" />
    </staticContent>
  </system.webServer>
</configuration>