I recently made a handy printed sheet of templates to help me center the screw pilot holes for 2x4 and 2x6 butt joints:
The idea behind these templates is that you cut one out from the printout, lay it where you want to drill the pilot holes for a butt joint, then use a spring-loaded punch through the paper to mark the workpiece. These can be used for 4x4s as well - I used a 2x4 template twice to drill the four holes for my 4x4 mailbox arm:
The templates are reusable until you lose them, they get too crumpled to work with, or the hole in the paper gets too hogged out. A PDF file is attached to this message. When you print it, BE SURE TO CHANGE THE SCALE TO 100% rather than "fit to page"! You want to be sure the PDF is printed 100% to scale, rather than squeezed into the margins of your current PDF printing settings.
[The extension pdf has been deactivated and can no longer be displayed.]