what is the reason you need the 4 extra volts?
if you want to run a 60volt lead acid system, you have to push the battery volts up to 75 volts to get them to equalize.
It sounds like the unit will need reprogramming, not merely the voltage limit changed. you may be able to find where it senses the battery voltage and adjust some resistors, but that may break something else, such as the battery recharge algorithm.