You want that other "Button 4" to have a different functionality or simply change its label.
For changing label use the set methods of the JButton class API.
For changing the button, you can try to look at the API of JButton, if it has a setVisible method.
You can have both buttons placed at that location but one would be hidden. When you click the "1" you can hide that and make the "4" visible.
Like I said in order for that to work you need to look at the API. I am not sure if it can be done