Why are there 4 terminals on a dryer thermostat?

Two terminals for the single pole thermostat switch and two terminals for the bias heater. When the bias heater is operating the thermostat will switch at a lower temperature - enables Low Heat (with bias) / High Heat (without bias)

http://store.valueweb.com/servlet/zapplian/-strse-1367/Dryer-thermostat-cold-control/Detail

Clothes dryer cycling thermostat with internal bias heater. Whirlpool

http://www.appliance411.com/parts/showcase.dryer-thermo.shtml

"The bias thermostat is just like a regular thermostat except it is located inside a bias heater.When the heater is energized it generates a little amount of heat which causes the operating thermostat to open sooner keeping the dryer drum cooler inside therefore a bias heater allows a single operating thermostat to act both as a hi temp and lo temp thermostat."

Peccavi
July 2009