Three-Electode vs Four-Electrode Experiments

What is the difference between electrochemical 3 probe measurements compared to 4-probe conductivity measurement?

Potential drop inside the device (solid state or electrolyte). AB and DE potential are caused by the contact resistance, when the current is running from A to B (Ohms law, U = IR).













Typical 3-probe setup in electrochemical cell. It is assumed that BC voltage drop is negligible and then AB potential is measured, which is a potential of interest on the working electrode.













Typical 4-probe conductivity measurements.Potential between B and D is measured in case there are significant AB and DE voltage drops. Important when contact resistance is comparable or larger than the resistance of the bulk, or when the small current is pumped (given that R_contact is independent of the magnitude of this current).
Pics are taken from Gamry Instruments.

And this is how to convert Keithley 2450 for 3-probe electrochemical setup