My home A/C unit is 4 years old just replace the coil last year, now the same unit want cool down the temperature after a few hours running. I went to flip the circuit breaker and notice it was tripped so a tried to turn it back on it would stay, so I cut off the air conditioner for now. What could be wrong?

You either have something wrong that's drawing too much power... or you have a defective breaker. Problem is either one is above your ability to safely play with or diagnose. I'd call an HVAC guy and have him check it out... he can replace the breaker for you if that's the fault.