Try to CAREFULLY pull the cable out of the wall to see if the installer just folded the 5th wire back (smart) instead of snipping it off (dumb but typical), you might still have to connect the 5th wire to the 24VAC common at the furnace.
CAUTION: If you have to disconnect the wires from the thermostat, be sure to tie a "retrieval" string to it first so it can't "escape" back into the wall.