Ohms: How do you decide whether to use a two-probe method or a four-probe method for resistance?

The decision will depend on how much lead resistance is present in the leads, how much lead resistance can be tolerated, and the application. A 4-wire ohms measurement bypasses the voltage drop in the test leads by bringing two high impedance voltage sense leads out to the unknown resistance. With very little current flowing into the sense leads, the voltage seen by the sense terminals is the same as the voltage developed across the unknown resistance.