The number of wires you need is dependent on the system you’re running with your ecobee4. Below are the number of wires you’ll need for each system: =
Heat-Only: You will need 3 wires (no Power Extender Kit)
Cool-Only: You will need 3 wires (with Power Extender Kit)
1-Heat + 1-Cool: You will need 4 wires (with Power Extender Kit)
The Power Extender Kit (PEK) is included in the ecobee4 box and makes it easy to install ecobee without running extra wires from your HVAC to the thermostat location.