Question on Smart Thermostat Wiring

https://lukaseolt009.swiftnestly.com/posts/what-you-can-fix-yourself-and-when-north-fort-worth-ac-repair-needs-comfort-experts

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