The fourth series of the British drama series Merlin began on 1 October 2011 with the episode "The Darkest Hour - Part 1". It consists of 13 episodes originally shown on Saturday evenings on BBC One and BBC One HD (repeats shown on BBC Three). The series producer was Sara Hamill, and executive producers were Johnny Capps and Julian Murphy. The directors of the series include Alice Troughton, Alex Pillai, Justin Molotnikov and Jeremy Webb. Writers include Julian Jones (head writer), Howard Overman, Jake Michie, Lucy Watkins, and Richard McBrien.
Series four stars the regular cast from the previous series including Colin Morgan, Angel Coulby, Bradley James, Katie McGrath, and Richard Wilson. Credited as a regular, Anthony Head left the series after his character was killed off in "The Wicked Day". Nathaniel Parker joined the regular cast in this series (credited as such for the first time in "Aithusa") and John Hurt returned as the voice of the Great Dragon. Supporting cast members include Adetomiwa Edun, Eoin Macken, Tom Hopper, and Rupert Young.
Notable guest actors include Santiago Cabrera, Emilia Fox, Gemma Jones, Phil Davis, James Callis, Lindsay Duncan, Gary Lewis, Charlene McKenna, Janet Montgomery, Terence Maynard, Ben Daniels, Miranda Raison and Caroline Faber.