The three wire RTD has two 'active' wires and one for compensation. The compensation wire is actually connected parallel with one of the 'active' wires to one end of the resistor, so those wires should read a small amount of resistance at the end where you connecting to the measuring device. This is used to compensate the measurement that may be introduced by the resistance of the home-run wire. The 4 wire RTD has a compensating wire connected to BOTH end of the resistor to compensate for the resistance AND also the difference that results in miniscule resistance difference that comes from manufacturing tolerance difference between the two leads. So you just hook up the 4 wire as a 3 wire RTD and leave one of the compensation wire unconnected.
FWIW, the three wire RTDs (and for that matter the two wire ones also) are usually sensed with a resistance bridge circuit containing known resistors. In such a circuit the additional (fourth) compensating lead is not really useful and its presence will not allow any more accurate measurement.
But using a four wire RTD, the four wires can be used as separate current and voltage leads allowing the resistance to be measured by looking directly at the measured voltage on the voltage leads while sending a known current through the current leads. In this situation, the lead resistance is simply not part of the measurement in the first place and no compensation is needed.
Bottom line: A four wire RTD detector can only be used with a four wire RTD probe, while a four wire RTD probe can also be used with a three or two wire sensing circuit.