SHORTLY AFTER TAKEOFF, THE SURVIVING PAX (S/P) NOTICED THE ACFT BEGAN LOSING ALT. SHE HEARD THE PLT COMMENT ABOUT A 'CLICKING NOISE IN THE LEFT WING' WHICH SHE ALSO HEARD & SAID SOUNDED LIKE METAL CLICKING TOGETHER. GROUND WITNESSES NOTICED A CHANGE IN ENG SOUND. SEVERAL RPRTD AN INTERRUPTION OF ENG PWR, BUT OTHERS (INCLUDING THE S/P) DID NOT RPRT A A PWR LOSS. SUBSEQUENTLY, THE ACFT CRASHED APRX 600 YDS SSE FM THE APCH END OF RWY 29. DRG IMPACT, THE LEFT WING & NOSE WERE EXTENSIVELY DMGD. AFTER IMPACT, THE ACFT TRAVELED ONLY ABOUT 37' BEFORE COMING TO REST. AN EXAM REVEALED THE #4 EXHAUST PIPE FAILED NEXT TO THE WELDED FLANGE THAT ATTACHED THE PIPE TO THE EXHAUST PORT. THE FRACTURES WERE DISCOLORED BY EXHAUST GASES & PEENED BY REPEATED CTC WITH THE MATING SURFACES, MAKING EVALUATION DIFFICULT. MUCH OF THE FRACTURE WAS 90 DEG TO THE PIPE SURFACE (TYPICAL OF FATIGUE). THE FLANGE HAD BEEN PREVIOUSLY REPAIRED BY PUTTING A SLEEVE OF MATERIAL ARND THE PIPE & WELDING IT TO THE FLANGE. SOME AREAS OF THE REPAIR WELD DID NOT ADHERE TO THE SLEEVE OR FLANGE.