Four wire motors are driven by a pair of H-bridges, look up the datasheet for the driver ICs being used. You can scope any one of the four wires relative to ground and you should see something however stepper motors are inductive and there is also magnetic coupling between the windings so you're likely to see a whole lot of noise and ringing. Better may be to look at the signals going *in* to the drive ICs and check the power rails. If the signals going in look good, the ICs have power, and there is not an open circuit between the IC and the motor and the motor is not running then the IC is likely bad.