The Fourth Season begins with Camelot being ruled by Prince Arthur and his shady uncle, Agravaine, who is secretly in cahoots with Morgana to take over the Kingdom. King Uther is non-functional from Morgan's betrayal a year before and can do little but sit in his room and stare out the window.
When Morgana takes her her dying sister, the evil Morgause, to the Isle of the Blessed a hole is ripped into the fabric of the spirit world, releasing these nasty skeletal creatures called the Dorocha. Their touch is death, so when they begin to attack Camelot there's lots of panic. Gaius, Merlin's mentor, reasons that the only way to close this hole is someone else dying. Merlin is touched by the Dorocha is dying. But Lancelot returns and sacrifices himself to save everyone, including the young sorcerer.
King Uther Dies and Arthur and Gwen get serious with their relationship. Merlin continues to secretly protect the kingdom and Arthur's life. In fact, he changes his appearance to that of an old white bearded man to throw people off his trail. Things get more complicated when the last remaining dragon's egg is discovered. Since Merlin has a connection to the dragons he feels it's his responsibility to save this creature--and ends up taking the egg to The Dragon (John Hurt) he gets his magical advice from. It's hinted that there may be dire consequences but he feels he has to protect these magical creatures.
Morgana is still plotting Arthur's downfall, along with the help of slimy Agravaine. Merlin discovers that the regent is a traitor but can't prove it. Things get worse when Lancelot returns from the land of the dead. Morgana puts a spell on Gwen that makes her betray Arthur to Lancelot and the new King is so upset that he banishes her from the kingdom. In the next episode, which involves a ghost, Gwen's brother, who is one of Arthur's knights, gets possessed by the spirit of a boy who wants revenge for his death. The King's demise is yet again averted with the help of his servant. Eventually, Arthur and Gwen are reunited, though he almost slays her when Morgana transforms her into a deer during a royal hunt.
The season ends with Arthur pulling the sword Excalibur from the stone, secretly assisted with the secret help of Merlin's magic. And Merlin defeats Morgana's invading army by calling forth the Dragon. This Fourth season is better than three, primarily because Arthur finally becomes King of Camelot.