Dec 18 (Reuters) - Canada’s main stock index opened higher on Friday, extending gains for a fourth straight session, helped by strength in energy stocks and upbeat domestic retail sales data. * At 09:30 a.m. ET (14:30 GMT), the Toronto Stock Exchange’s S&P/TSX composite index was up 18.5 points, or 0.1%, at 17,671.44. (Reporting by Amal S in Bengaluru; Editing by Ramakrishnan M.)