gas
Which kind of torque are we talking about. Are we talking about "holding torque" where the motor is not moving, or are we talking about torque at some speed. All stepper motors have resonances where the torque drops abruptly to zero. You can get around this problem by ramping the velocity up and back down again. Are you doing this? There is also an upper speed limit where the torque is zero and there is no workaround.
reyandy
A stepper with four wires is probably a bipolar stepper motor. Searh this and other forums for threads on how to drive them.