I am not certain what your design is. The deault program is designed to handle 4 joysticks. If the purpose is to have a secondary drive system, just use 3 and 4 to control the other motors(use 2nd driver) , or use the left=right axis on the joysticks to control the other motors. THis would take a 1 co-ordinated driver.

If you want to control 4 motors with just two joysticks and say the
trigger switches, you would need to reprogram. It is hard to get
four motors that are not connected to work in concert. Good luck.