I don't think there will be a reveal about the Gwen Lancelot Bracelet enchantment. I think the writers will use Arthur's doubts to further test his relationship with Gwen in series 5 (the course of true love never does run smooth) although I think it is part of Arthur's nature to be forgiving (unlike his father) and I don't think he needs proof of Gwen's enchantment to forgive her for her indiscretion with Lancelot.
If it is the last series, there must be an accidental reveal of Merlin's magic to Arthur and I think it will bring a conflict between them that will be resolved later. Arthur will not take it lightly that he was deceived by Merlin and was the last to know about his magical powers. I think it likely that Gwen will discover Merlin's secret first.
I think there will be a big magical duel between Morgana and Merlin with Merlin being victorious (of course) and this will reassure Arthur that although magic can corrupt and be used for evil (as in Morgana's case), in the hands of someone good and pure of heart like Merlin, it can also be used as a force for good.
Maybe the writers will take a big departure from the Arthurian legend and give Arthur and Gwen a child. This could introduce the idea that Arthur is conflicted as whilst delighted at the prospect of an heir, he is also fearful for Gwen's safety as he is reminded of his own mother's death in childbirth.
Just a few ideas that may come into the series. On the subject of the Knights, might we finally meet Sir Gallahad? Lancelot's departure has left a hole which could be filled by the introduction of the most gallant knight in the Arthurian legend of old.