Contact resistance isn't eliminated with four terminal resistors, it is mitigated. Basically, there are four connections directly on the sense resistor, one pair carry the working current, the other pair normally have a high resistance measuring circuit on them, an amplifier or voltmeter, that draws a current very much less than the working current. This, therefore, will measure the voltage directly across the resistor, ignoring the voltage drop across any leads and terminals to which the resistor may be connected.
Contact resistance is a whole new subject, around mechanical and electro-chemical issues for starters.
Some useful sites below.
http://www.allaboutcircuits.com/vol_1/chpt_8/9.htmlhttp://www.mouser.com/pdfDocs/Ohmite_CSResistors.pdfhttp://en.wikipedia.org/wiki/Four-terminal_sensingCheers,
Richard
