A catheter and patch electrode system is provided for use with an apparatus, such as an ablation generator, having a 4-wire interface for improved impedance measurement. The 4-wire interface includes a pair of source connectors across which an excitation signal is produced and a pair of sense connector wires across which the impedance is measured. The RF ablation generator may also produce an ablation signal across a source wire and an indifferent return patch electrode. The system further includes a cable that connects the generator to a catheter. The catheter includes a shaft having a proximal end and a distal end, with an ablation tip electrode disposed at the distal end. A source lead is electrically coupled to the tip electrode and extends through the shaft to the proximal end where it is terminated. An optional sense lead is also electrically coupled to the tip electrode and extends through the shaft to the proximal end. The system further includes a source return (e.g., skin patch) and a sense return (e.g., skin patch), either or none of which may be combined with the indifferent return, and if used may be placed on opposite sides of the patient for improved performance. The impedance sensor circuit produces an excitation signal across the source connectors, which is then carried to the catheter by the cable, then to the tip electrode, travels through the complex load (tissue volume), and returns to the generator via a patch electrode. The impedance is measured by observing the voltage drop across the sense connectors caused by the excitation signal.