The car was located by a neighbouring landowner about 5:00pm and appeared to have rolled down an embankment, police said.
Police were told the three men and the boy had travelled to the property off Waverley Road, Gundy, this morning.
Concerns were raised for their welfare this afternoon after they failed to return.
The boy survived the accident with minor injuries including a broken arm, and was found by people on the property.
“The people on the property … have come across the young boy and emergency services have been called,” Glen Ramplin from the Westpac Rescue Helicopter service said.
The boy was airlifted to flown to the Scone Hospital before being taken by ambulance to the John Hunter Hospital in Newcastle, where he remains in a stable condition.
“He’s obviously a tough little boy,” Mr Ramplin said.
Police said it was believed the accident happened earlier today, but the alarm was only raised when the boy was found.
All occupants had been ejected from the vehicle, and the three men were located dead at the scene, police said. Police will prepare a report for the Coroner.