New and exciting teasers have been posted concerning ABC Network's Once Upon a Time Season 4 which is set to air this September. It is already known that Queen Elsa of Frozen will be joining the cast in the upcoming installment, but what fans don't know is that "Captain Swan's" story will result in some major plot twists.

Emma Swan and Captain Hook's seemingly happy ending last season will unfortunately see some challenges in the upcoming Season 4. According to co-creator Adam Horowitz, "The very first obstacle is Emma's guilt over the fact that she just destroyed Regina's relationship [with Robin Hood], so there's a part of her that feels guilt in starting a new romance of her own," he said to TVLine.

Also, Emma and Regina's rivalry will be fiercer and involve many cat fights this season. Regina will surely hold a grudge against Emma for ruining her chances of bringing back Maid Marian along with her happiness. The upcoming installment should procure some interesting advancements between these two major characters.

"Emma definitely wants Regina to be with Robin, and she never would have intentionally done anything to hurt that. But she'd also never let someone die in the circumstance that [Marian] was in," Emma's Jennifer Morrison told TVLine.

Meanwhile, as the fans are excited to see actresses Kristin Bell as Anna and Idina Menzel as Elsa, the production team has opened their doors for casting on additional roles. One of their more important duties will be find and fit someone with the Ice Queen role in the show.

Once upon a Time Season 4 will return this fall on ABC with 8/7c timeslot.