WHILE ABV A CLOUD LAYER (TOP AT FL 370) NR THE JET STREAM, AUTOPLT WAS ENGAGED & WAS IN THE PERFORMANCE MNGMNT SYS (PMS)MODE. THE PMS PROVIDED PITCH GUIDANCE TO HOLD FL 410, ROLL GUIDANCE TO AILERONS & SPOILERS FOR ROLL CTL & AUTOTHROTTLE TO MAINT .85 MACH (254 KIAS). ACFT ENCOUNTERED CLR AIR TURBC & AIRSPEED BGN TO VARY BTN .84 & .88 MACH. PMS BGN MOVING THROTTLES FORE & AFT TO HOLD .85 MACH. DRG AUTOTHROTTLE ADJUSTMENTS, #4 ENG THRUST DECREASED & 'HUNG' AT APRX 1.0 EPR & AIRSPEED BGN DECREASING. FLT ENGINEER ATMTD TO MANUALLY RCVR THE #4 ENG THRUST, BUT DID NOT CLOSE THE BLEED AIR VLV BFR ADJUSTING THE #4 THROTTLE. THE #4 ENG REMAINED AT APRX 1.0 EPR. AS THE ACFT SLOWED, AUTOPLT TRIMMED TO HOLD ALT & HDG TIL THE PLT DISENGAGED IT. AT THAT TIME, ACFT ROLLED/YAWED RGT & ENTERED AN UNCTLD DSCNT INTO THE CLOUDS. AS IT BROKE OUT OF THE CLOUDS AT 11,000', CREW RCVRD & LVLD AT 9500'. DRG DSCNT/RCVRY, ACFT WAS DMGD BY ACCELLERATION FORCES & HI SPEED. THERE WAS EVIDENCE THE PLT WAS PREOCCUPIED WITH ENG PRBLM, DIDN'T MONITOR INSTRUMENTS & OVER-RELIED ON AUTOPLT.