Difference between a single-pole thermostat and a
double-pole thermostat

Single-pole thermostats (2-wires) cut off the power on only one side of the power wire. Therefore, the thermostat and the heaters remain powered at all times.

Double-pole thermostats (4-wires) cut both sides of the power line. When the thermostat does not ask the heaters to work, the power to these devices is completely cut off.