Question on Smart Thermostat Wiring

https://manuelvwth616.keystonescope.com/posts/why-north-fort-worth-homeowners-choose-a-local-company-like-comfort-experts-for-ac-repair

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