Question on Smart Thermostat Wiring

https://threadvistax.com/s/Lv1nh2j6qczUftPNcURO1

Question on Smart Thermostat Wiring Our older thermostat only has four wires and no C wire. The smart model I bought says it needs one. Is an adapter safe to use, or does that cause problems with the AC down the road? Trying to avoid a wiring mistake