Still, the biggest change for installation is that the Nest Learning Thermostat (4th gen) does not need a C wire for power. Instead, it draws power from all the wires to charge the battery.

The Nest Learning Thermostat 4th Gen includes a Power Sharing feature, utilizing power from your existing thermostat wiring.

That's a pro and a con really: although I've been using the system for almost two years and have no low-battery ... if the Smart Thermostat outlay is a little pricier than Nest, Tado or Hive ...