What was found
(CAN) UPON SELECTION OF FLAPS 15 ON LANDING CHECKLIST THE L/E FLAP TRANSIT REMAINED ILLUMINATED AND THE NR 4 SLAT INDICATOR WAS SHOWING IN TRANSIT. REPLACED AND ADJUSTED NR 4 SLAT PROX SENSOR IAW MM 27-88-11. NR 4 SLAT FOUND DENTED ON UPPER SURFACE, REPLACED NR 4 SLAT IAW MM 27-81-21. RII CARRIED OUT. UPON FURTHER INVESTIGATION IT WAS FOUND THAT DAMAGE FROM A PREVIOUS BIRD STRIKE WAS FOUND TO THE SLAT CURTAINS WHICH CAUSED THE DAMAGE TO THE PROX SENSOR. THIS DAMAGE WAS NOT VISIBLE WITHOUT REMOVING L/E PANELS.