Four members of a family, including an elderly couple, were severely injured when a domestic gas cylinder in their house caught fire and exploded here early this morning, an official said. Due to the impact of the explosion, a wall of the house collapsed, he said. There was some leakage in the pipe of the LPG cylinder because of which it caught fire and exploded around 6.30 am in the house located at Siddeshwar Talao in the city's Patil Wadi area, Thane Municipal Corporation's regional disaster management cell (RDMC) chief Santosh Kadam said.