Question on Smart Thermostat Wiring

https://beausrhq666.juniperbrief.com/posts/timberland-ac-repair-in-north-fort-worth-and-why-comfort-experts-is-close-by-4

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