It returns six months after the shocking finale of Season 3, and the team must overcome their grief to tackle a dangerous crime wave.
This time, the gloves are well and truly off for Professor T in his lectures and a new musical pursuit, romance is very much alive for his mother Adelaide Tempest (De la Tour), and the lines between professional and personal become blurred for therapist Helena Goldberg (Stevenson).
Meanwhile, DI Maiya Goswami (Sunetra Sarker) takes charge of the police force with series favorite DS Dan Winters (White) fiercely at her side.