From the BBC web site
Saturday night family hit Merlin is to return for a fourth series, BBC One bosses have announced. The third series of the retelling of the Arthurian legend has regularly attracted average audiences of more than six million. "Merlin continues to perform outstandingly well against The X Factor," the BBC's controller of drama commissioning Ben Stephenson said. The show stars Colin Morgan, John Hurt, Richard Wilson and Anthony Head. The BBC said the show had been sold to 180 countries. The third series, which ends on 4 December, has featured guest appearances from actors including Warwick Davies and Miriam Margolyes. Filming for the new 10-part series will begin in March.