In four-electrode mode the Working Sense lead is decoupled from the working electrode, as was (and in addition to) the Reference lead (see Figure 5).
Four-electrode setups measure potential along the B-D line in Figure 3, where there may be some “obstruction” at C. This setup is relatively uncommon in electrochemistry, though it does have its place. In 4-electrode mode, the potentials for any electrochemical reactions that are occurring at the working (and counter) electrode(s) are not being measured. What is being measured is the effect of an applied current on the solution itself or some barrier in that solution