On most older A/C units you are correct that the fan and compressor would continue to run even if the unit was out of refrigerant. On many newer units a low pressure switch is used and that switch when open will keep the compressor and fan outdoors from running. But still, the fix is not a part usually. If there is a leak it needs to be repaired. Sometimes parts do develop leaks and the fix is to replace the part, like a reversing valve on a heat pump. And. nothing at all unusual about a 4 year old system having a failure. I always recommend the simplest builder grade system available. Carrier, Trane,Lenox are loaded up with unnecessary crap that cause more system failures. Kinda like all new Ford's, Dodge's and Chevy's. Buy Toyota.