When you install your Nest Thermostat without the common wire, electricity is not continuous. Meaning that your thermostat will have to rely on the battery for power when the thermostat is not activated in heating or cooling mode.
To allow for continuous power, you must install a (fifth wire) common wire. As a nest Certified Professional we make it a point to supply
Nest says, ” In most cases, Nest thermostats can charge their built-in battery using your system’s heating and cooling wires.”
But sometimes a common wire is required to ensure continuous operation.
You can read more about Nest Common Wire Here: