4 Wire or 5 Wire Thermostat Wiring Problem - Wifi Tstat 4 Wire or 5 Wire Thermostat Wiring Problem – Question: We have a 19-year-old condo with an old Carrier mercury thermostat. We have both a natural gas furnace and an AC unit. There are only four wires going into the existing thermostat – red, green, blue, white. Thermostat Wiring for Dummies – How Anyone Can Do It Heck, now I’m not calling anyone a dummy! Even the most seasoned people who enjoy DIY may come across problems with wiring a thermostat. The main trouble is dealing with those different colored wires, knowing which one does what and if that wasn’t enough, you also have numbers and letters to deal with.