What was found
DURING FLIGHT INTO MIA, FLIGHT 203, COMPLETE LOSS OF NR 4 HYD SYS, BOTH FLUID AND PRESSURE, AS INDICATED BY QTY GAUGE, LOW QUANTITY LIGHT ILLUM. ZERO PRESSURE INDICATED ON GAUGE AND ILLUMINATION OF BOTH ADP AND EDP PRESSURE LIGHTS ON F/E PANEL AND ON FWD INST PANEL. FOUND HYD SHUT OFF VALVE SYSTEM NR 4 IN TAIL COMPARTMENT FAILED. REMOVED AND REPLACED VALVE IAW MM 27-28-00. OPS CHECK AND LEAK CHECK OK. RESERVICED NR 4 HYD SYSTEM.