DCA83AA029

During a cruise climb, the aircrew noted an unusual vibration; however, attempts to isolate the vibration were unsuccessful. While climbing from FL190 to FL250, the flt engineer went aft to observe the engs & props, but noted nothing unusual. A flt attendant went forward to advise the capt that nothing was observed, when the vibration increased in intensity. As she left the cockpit, she peered out the rgt window & saw the #4 prop separate & strike the acft. This caused major damage to the lower fuselage & resulted in decompression of the cabin. The flt controls became jammed & the engs would not respond to throttle movement. The autoplt was used for minimal control & the flt crew were able to divert to anchorage, AK. They were able to climb & descend by retracting & extending the landing gear with the #2 eng shut down. The acft was landed safety & the engs were shut down to decelerate . The exact cause of the prop separation was not determined since it fell in the ocean during the occurrence.