How do i hook up a new 5 wire cable to an existing 4 wire furnace
The thin blue wire at the thermostat is connected to the Y terminal, which means it is used to call for cool (air conditioning).  You'll notice at the furnace, that the thin blue wire connects to the thin red wire. The thin red wire goes to the contactor coil in the A/C unit outside.


Once you have the new cable run, you'll connect the wires as described below.

At the furnace

Remove the old thermostat wires (leaving the wires for the A/C unit alone).
Connect the green wire from the new thermostat cable, to the thick green wire from the furnace.
Connect the red wire from the new thermostat cable, to the thick red wire from the furnace.
Connect the white wire from the new thermostat cable, to the thick white wire from the furnace.
Connect the yellow wire from the new thermostat cable, to the thin red wire from the A/C unit.
Connect the blue wire from the new thermostat cable, to the thick blue wire from the furnace and the thin white wire from the A/C unit.

At the thermostat

Connect the red wire to R
Connect the green wire to G
Connect the white wire to W
Connect the yellow wire to Y
Connect the blue wire to C