In a four wire configuration, the control voltage is the voltage between {HEAD ~ a neutral point} formed by three unequal resistors and {TAIL ~ the neutral line} 
In a four wire configuration , the control voltage is the voltage between { HEAD ~ a neutral point } formed by three unequal resistors and { TAIL ~ the neutral line }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL