Q: How download from EMAIL a video in MPEG 4 format?

Hello, actionjw.  

It sounds like you want to download a video that was sent to you via email in a particular format.  Looking at the tags on this post, are you running 10.4.11 or a newer OS X version?  While there isn't a way to download a mail attachment as a particular media type as the video container is determined by the sender, you may want to look for third party software that would convert this file to the desired type.  These software options are only advised for non copyrighted material.  If you are unfamiliar with how to save a mail attachment, I have attached an article that will guide you in that process if unfamiliar.  The article is labeled for Mavericks, but will also apply to Tiger.  

Mail (Mavericks): View, save, and delete attachments

If you are experiencing issues playing a file, could you provide more details on what exact issue you are experiencing?  Could you also confirm what Mac OS X version you are running and what is the current type of the file in question? 

Find the OS X version and build number on your Mac

Thank you for visiting Apple Support Communities.  

Cheers