So, your voltage is 48V (4x12v pb battery - maximul voltage 13,5-13,8V fully charge = 55,2V )
Liion batteries fully charged 4,2V so 55,2/4,2= 13S battery. ( in that case, you will not fry your controller)
but problem is fully discharged PB battery is 11,6V (46,4V) and liions fully discharged...