On February 15, 2015, about 0103 coordinated universal time, United Airlines flight 15, a Boeing 767-400ER, N59053, encountered severe turbulence over the Pacific Ocean while enroute to Honolulu International Airport (HNL), Honolulu, Hawaii. Of the 250 passengers and crew onboard, one flight attendant was seriously injured, and three flight attendants and one passenger sustained minor injuries. The airplane was not damaged. The flight was operating under 14 Code of Federal Regulations Part 121 as a regularly scheduled passenger flight from Newark Liberty International Airport (EWR), Newark, New Jersey, to HNL. The flight was uneventful from EWR across the continental United States. Once over the Pacific Ocean, the flight received an ACARS message from dispatch advising of an area of possible embedded thunderstorms. The flight experienced light to moderate turbulence for most of the remaining flight, which required the seat belt sign to remain on most of the time. About an hour from HNL, the captain returned early from his crew rest time because he noticed that the flight was encountering areas of moderate turbulence. Prior to entering the flight deck, the captain noticed that the flight attendants (FA) were seated in their jumpseats and advised the lead FA to not conduct the final service and remain seated. Shortly after retaking the left seat, and as the flight was passing immediately south of a line of embedded of the forecasted thunderstorms, the captain requested a descent from their cruising altitude of flight level (FL)360 to FL240. The flight crew reported they were in instrument meteorological conditions with precipitation during the descent. The weather radar was on and the seatbelt sign was illuminated. The flight encountered an area of severe turbulence for about 30 seconds as it was descending through FL254. The flight data recorder data indicate that the vertical acceleration fluctuated between about -0.17 g's and +1.62 g's, the lateral acceleration fluctuated between about -0.09 g's and +0.11 g's, and the roll fluctuated between about 5° left and 12° right. At the time of the turbulence, the FAs were securing the galleys, preparing the cabin for landing, and distributing agriculture documents. Four flight attendants (FA) in the aft galley were thrown against the ceiling and back to the floor, injuring all four. After being notified of the serious injuries, the flight crew declared an emergency with air traffic control and arranged for medical personnel to meet the aircraft at HNL. The four flight attendants were transported to the hospital where one flight attendant was diagnosed with a fractured arm and the other three were released with minor injuries. After landing the crew were notified by a passenger that their child had suffered a bloody nose, who was treated but not transported to the hospital.