How to map a custom MS Word keyboard shortcut to a button and add it to the quick access toolbar on the ribbon?

Question

I have several custom keyboard shortcuts for styles that I want to map to buttons on the Word ribbon.

For example, my custom keyboard shortcut for a Heading 4 is Ctrl-Alt-4. I managed to find and add Word's built-in buttons for H1, H2, and H3. There is no H4 button. So, how do I:

1. Find or craft a custom button?
2. Map the keyboard shortcut to the button?
3. Add the button to the ribbon toolbar? or
4. Is what I want even possible?

Detailed instructions or links to detailed instructions will be helpful (versus 30K views).

Note: Yes, I know I can use the gallery. I want compact buttons for frequently used styles, not gallery thumbnails.