The chart on the motiononline.com datasheet (page 6) says that you need DC24V to operate the 2-Phase driver. The high voltage is for the driver circuitry, not the 4V stepper.
Anyway, the 24V is applied to the driver, and it steps down the voltage to drive your stepper. Check the pintout schematic on the driver to make sure.
. . . but in my humble opinion its kind of lame to carry 24V batteries on a robot for a 4V stepper . . . I guess they designed the driver that way to handle many different types of stepper motors.
You should consider maybe using a stepper driver IC that operates around the 4-7V range.