As I show you in my books, it's easy to output custom videos for your web site. You say want an MP4?
Go to Publish & Share/Computer/AVCHD. Select the the Preset for Vimeo SD.
Then click the Advanced button. On the Export Settings screen that opens, go to the Video tab. Set the output size to 320x240, then click OK.
You'll be prompted to save this as a "permanent" preset so you can use it again. Type in a name like "Web site video" and you're done.