In Xcode 6 how do I make it so only one of four checkboxes can be selected at one time?

I have an OS X app with four primary buttons to select actions and four checkboxes (one for each button) that makes the corresponding button's action the default so the action selection is skipped and the selected action begins immediately.

I can not figure out how to make it so that only one of the "Make Default Action" checkboxes can be selected at any time?