Updated at 11.28am:\nFour firefighters were injured when their fire engine overturned on a Chek Lap Kok runway on Tuesday morning. The vehicle, used for spraying foam, was driving on southern runway V4 when it suddenly flipped over around 7am, a Fire Department spokesman said. The cause of the accident is not yet known. ''Police are investigating whether the vehicle flipped because of a sharp turn, high speed, or other factors,'' the spokesman said. The four firefighters sustained slight injuries and were discharged from Princess Margaret Hospital after treatment.