Question on Smart Thermostat Wiring

https://cesarvmck284.brightpathdigest.com/posts/comfort-experts-ac-repair-tips-for-hot-upstairs-rooms-in-north-fort-worth-houses-3

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