How do I add a bipolar stepper motor to this circuit?
You cannot just connect it to your Q4 to get any useful behavior.  You need to do one of two things: get a stepper controller (or controller + driver), and drive it's enable, direction, and clock pins from three pins on the microcontroller.  Or, get a stepper driver and drive its enable, phase A, and phase B pins from the microcontroller.  Either way you need to understand the needs of the stepper (or controller) and meet them.