What was found
(CAN) AFTER TAKE OFF, PILOT EXPERIENCED DIFFICULTY CONTROLLING NR 4 PROP RPM. AFTER INFLIGHT SHUT DOWN, PILOT UNABLE TO FEATHER NR 4 PROPELLER. UPON INSPECTION, NR 4 PROP FEATHERING SWITCH WAS FOUND TO HAVE FAILED INTERNALLY IN SUCH A MANNER THAT IT WAS TURNING THE PROP FEATHERING PUMP ON/OFF UNCOMMANDED. THIS SUBSEQUENTLY CAUSED THE RPM CONTROL DIFFICULTY WITH THE GOVERNOR. THE PROP FEATHERING PUMP MOTOR FAILED DUE TO OVER- EXTENDED USE, WHICH CAUSED THE PILOT AN INABILITY TO FEATHER THE ENGINE. BOTH COMPONENTS WERE REPLACED AND AIRCRAFT EXTENSIVELY TESTED PRIOR TO TEST FLIGHT.