I am trying to figure out how to create a new keyboard shortcut in Word (using Office for Mac 2011). When I say "new," I mean a shortcut not listed under Tools>Customize Keyboard.

Specifically, I want a shortcut for applying the "Heading 4" format. The shortcuts for ApplyHeading1, ApplyHeading2, and ApplyHeading3 are all listed. But ApplyHeading4, 5, etc. are not listed and I can't figure out how to add them. Really, I can't figure out how to add a shortcut that isn't already listed.

I would really appreciate any advice on how to do this.