A common wire (C) is required for 5--wire thermostats. If there are only four wires to your existing thermostat (i.e. there is no C wire), you can only use this Power Extender Kit (PEK) to power your TP-WEM01 or T6-WEM01 thermostat.
Warning: this product contains a chemical known to the State of California to cause cancer. P65Warnings.ca.gov