Screen4 is on the download page, near the bottom I think.
Just open your screenset in screen4, click on the Img button at the top to get rid of the main bitmap then click on either of the button icons on the left and then click on the screen where you want the button then click on the None icon on the left so you dont inadvertently add more buttons. Next double click your button and choose VB Button option. Save your new screen as something else, that way it wont get overwritten if you upgrade Mach. Next open Mach and view menu then load screens and choose your new screen, lastly you go to Operator menu then Edit Button Scripts, your button should be flashing, click it and in the editor that pops up put your script, and thats it done
Hood