First episode Will be Episode 1 in season 4
The Darkest Hour (Part 1)
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Episode 1, The Darkest Hour (Part 1) was fine just as it was. I thought the Callieach was a great character.

Episode 2, The Darkest Hour (Part 2) could be improved by having Merlin mutter Freya while he was being healed by the spirits leaving no doubt that she was still with him. And, I liked the fact that Lancelot sacrificed himself at the end.

Episode 3, The Wicked Day could have been better by putting in the part about how Arthur's not convinced that all who practice magic are evil.

Episode 4, Aithusa could have been better by giving more background about why a Druid symbol like the Triskelion being used as a key to keep the dragon's egg hidden.

Episode 5, His Father's Son could have been better by having Arthur reveal that he was having second doubts about much of what his father did including his attitude about magic.

Episode 6, A Servant of Two Masters could be improved by having Agravaine explain his loyalty to Morgana and justify her claim as the rightful heir to the throne of Camelot.

Episode 7, The Secret Sharer could be improved by having Gwaine assure Merlin that Gwaine doesn't think Gaius is a traitor and wants to find Gaius because Merlin's a friend.

Episode 8, Lamia could be improved by having the knights apologize to Merlin as they recovered shortly after Arthur's visit and having Merlin find a magical object that could be used against Lamia.

Episode 9, Lancelot du Lac could be improved by having, near the very end a guard find the enchanted bracelet and giving it to one of the serving girls as a present.

Episode 10, A Herald of a New Age could be improved by having everyone know that the Druid Shrine was the place where Arthur rescued Morgana and that that resulted in the death of innocent women and children.

Episode 11, The Hunter's Heart could be improved by having Merlin know that sorcery was used to duplicate the siege tunnels of Camelot document but unable to tell Arthur.

Episode 12, The Sword in the Stone (Part 1), could be improved by having Arthur tell Merlin about a story about a scabbard and sword cherished by the ancient kings while Arthur struggled to put on the belt... the Sword was put fast in a stone and the scabbard's whereabouts is a mystery... and Merlin telling Arthur that Gaius said something about that once, before putting another hole in the belt.

Episode 13, The Sword in the Stone (Part 2), could be improved by having Isolde dying, Tristan showing his love for her, Isolde surviving her death and through the joy and love of Tristan and Isolde, Arthur realizing that his love for Gwen was greater than "anything."