Unforgotten series four kicks off tonight (Monday, February 22) – and ahead of the premiere, the stars of the show have been talking about how important their chemistry is to the show's success.Nicola Walker and Sanjeev Bhaskar will reprise their roles of DCI Cassie Stuart and DI Sunny Khan respectively, as they try to unravel another cold case.It has already been confirmed that the fourth series of the BAFTA-nominated show will open with the discovery of a dissected body in a scrap metal yard,