Help! Onan 4K generator refuses to start and stay on

Does the generator keep running if you hold down the start switch? If it does, that indicates that it's not putting out enough voltage to the control board to lock in the run cycle. It could be a faulty voltage regulator ($$$), or it could be dirty slip rings. Holding the start button down for a minute or two may clean the rings enough to keep it running. Worth a try anyway...