Hey there! I's like to know how to replace an old white-rodgers mercury thermostat (Model 1E30W609) by a simple digital thermostat? The current mercury thermostat controls a natural gas fireplace. Im not sure which wire to connect to the new thermostat. Plain simple, why is there 4 wires ? A black, red, green & yellow... If I buy a replacement thermostat, I should look for a 4 wire right? Anything special I should know?? See pics! Thanks!!