4 wire stepper motor is a bipolar stepper motor which should be be driven in bipolar mode. Each phase has only one winding, therefore, the driving circuit is more complicated to reverse the pole, which is to reverse the current in the winding. In terms of structure, bipolar motors have multiple (at least two) independent windings. Each winding has a wire at the end, and each winding has two wires.
4 wire stepper motor has 2 leads in each phase, H Bridges with some driving topologies do have static friction effects, but this effect can be reduced by dithering stepper motor signals at higher frequencies. The stepper motor driver uses an h-bridge circuit to actually reverse the current flowing through the phase. All coils can be operated to rotate the stepper motor by alternating polarity excitation phase. Since all coils can be used, this stepper motor will get more torque.