What was found
BOI - AIR TURN BACK - FLT 0166 - ON DEPARTURE OUT OF BOI EN ROUTE TO PHX THE NR 4 SLAT REMAINED IN EXTENDED POSITION; FLAPS WERE CYCLED AND NR 4 SLAT STILL REMAINED EXTENDED; TURNED BACK TO BOI AND LANDED WITHOUT FURTHER EVENT. MAINTENANCE INSPECTED THE NR 4 SLAT AND FOUND SEVERAL ROLLERS STICKING DUE TO LACK OF LUBRICATION. ROLLERS AND TRACK WERE LUBRICATED, AND FLAP SYSTEM WAS CYCLED SEVERAL TIMES WITH NO DEFECTS NOTED. A FOLLOW-UP (FUP) WAS WRITTEN TO REPLACE THE NR 4 SLAT ACTUATOR AND LUBRICATE SLATS/FLAPS PER BMM 12-22. TEARDOWN OF NR 4 SLAT ACTUATOR S/N 1660T CONFIRMED FAILURE FINDING THE BEARING, SEALS, RINGS, O-RINGS, RETAINERS, AND SCRAPERS WERE WORN. UNIT WAS OVERHAULED.