We are going to change our furnace to a Rheem gas furnace. It comes with a Rheem modulating thermosat installtion. There needs to be 4 wires connecting the thermosat to the furnace. What do you do if you have 2 wires already? Also is there something I can use to control temperature with a sensor?

You don't have ac I take it. It should be able to run with just to wires if you get a batteried powered stat.