If you want to do 4-axis machining, simply mounting a stepper to your mill's bed isn't going to help much. You really need a rotary table with a substantial gear ratio to withstand cutting forces that will tend to overpower the stepper. G-code commands don't tell steppers to rotate continuously, although you can direct it to advance to a large number of degrees. If you wanted continuous motion, it would be better to set it up as a spindle rather than an axis, and use M-codes to control it, using PID to set the speed. Usually a different sort of motor would be used for that, not a stepper. Or you could start with a lathe rather than a mill or router.