My old thermostat had 4 wires. I replaced with a Nest thermostat. At times the AC condenser is running but the furnace blower is not running. Found other with this and they said need to add common wire. I do have an extra wire just not sure where to connect it. there is a C terminal on the furnace, just not sure if its to be used for the thermostat or not, it has 1 wire connected already. attached is a picture. Furnace is a Kelvinator g6rc-080c:
Simply land your blue wire on the C terminal
Most HVAC control board low voltage terminal blocks are designed to terminate 2 wires under each terminal, as was done with the Y terminal on your control board. So, turn off the furnace, loosen the screw on the C terminal, land the blue wire there (after stripping the end back if need be), tighten the screw back down, turn the furnace back on, and you'll be good to go.