1. Simple HTML 5 solution
Code:
<a href="path_to_mp4_file.mp4" download="filename.mp4">Click here to download</a>
This will force the download just as the user has right clicked and clicked save.
2. Much better, but bit complicated solution is to setup metadata on the object
Code:
Content-Type: "application/octet-stream"
Content-Disposition: "attachment"
You can do this trough S3 console...