A four wire RTD is the most accurate method to measure an RTD. It is primarily used in laboratories and is seldom seen in an industrial application.
The four-wire resistance thermometer configuration increases the accuracy and reliability of the resistance being measured: the resistance error due to lead wire resistance is zero.
Four wire RTD Circuit
A four wire RTD circuit removes the effect of mismatched resistances on the lead wires.
A constant current is passed through L1 and L4, L2 and L3 measure the voltage drop across the RTD element.
The color code for a four wire RTD is usually two red wires and two white wires.