As far as I can tell from a quick glance at a Halo board, there is a voltage regulator but only for the circuitry - the motor appears to be driven directly off the incoming battery connector. I'm pretty sure the motor is PWM-driven and used back EMF to measure motor load, but how well that compensates for varying supply voltage I'm not sure. Bear in mind that the motor itself is only intended for 3V operation, so it must use a fairly short duty-cycle, in which case I doubt the extra volt or so would make a big difference.