It has been confirmed that Mordred will return in Series 4 and it is possible that he will team up with Morgana to make Merlin pay for betraying them.
In the official Series 4 trailer Mordred is faintly heard saying "Emrys" when Merlin gets pushed against a wall by magic and it is possible that Mordred is the one who pushes Merlin against the wall to take his revenge on him for his betrayal.
So what may happen?
And do you think katie will let off spoilers like last time? :)
Merlin trailer will be posted in the videos later on tonight (: