If the stepper has only 4 wires then it must be a bipolar two phase motor. Use a multimeter to measure the resistance - this will determine the two wires for each coil. If it has five wires then you most likely have a 4 pole unipolar stepper motor, again use the multimeter to determine what is connected to what - the resistance is the lowest between the common and each phase. Phase to phase will be twice the resistance. Home do you know what phase is what? Guess! Or try to find the manufacturers datasheet. Otherwise you try all combinations to find which one works. If the combination is wrong, the motor will buzz rather than move.