Ohms: Which SourceMeter instrument do you recommend for four-point probe measurement techniques?

All Keithley Series 2400 SourceMeter instruments, as well as the Model 6430 Sub-Femtoamp Remote SourceMeter, are capable of making four-point probe measurements. That is, along with the input/output HI and LOW terminals, there is a corresponding SENSE HI and LOW that is used to correct for the lead wire resistance. It's important to consider the voltage and current capability and the measurement resolution in order to select the appropriate model.