REV4 boards need patch!
Warning: REV4 boards have a stepper ground problem: This can damage the MBED or stepper driver. you need to patch the board!
Due to a problem in routing the REV4 PCB boards, there is a part of the PCB that is not connected to ground. This can damage the MBED and/or pololu steppers. You need to add a jumper wire to fix the problem.