What was found
AIR TURNBACK, LOSS OF HYDRAULIC SYSTEM NR 4. CHECKED THE NR 4 ENG CASE DRAIN DPI NOT POPPED, AND REPLACED EDP AND ADP CASE DRAIN FILTER, NO METAL CHIP IAW AMM 29-11-18, 29-11-00, REPLACED THE LT WING MLG DOWN LOCK ACTUATOR, SERVICED HYD SYS 1, 2, 3, 4 CONDITION NORMAL, CHECKED, NO LEAK FOUND, NO FAULT FOUND IAWAMM 32-32-15, AND AMM 12-12-01.