The "Once Upon A Time" Season 4 premiere date on ABC has been announced, and filming is underway. Queen Elsa, portrayed by Georgina Haig, has been photographed in costume while filming. OUAT co-creator Adam Horowitz has also revealed on Twitter the title of the fourth season's first episode. And now a mystery for the fans: Which Frozen arc character will be portrayed by new cast Elizabeth Mitchell?

Spoiler alert: This feature contains Once Upon A Time Season 4 spoilers. Read at your own risk.

"A Tale of Two Sisters" is the title of the first episode of Once Upon A Time Season 4. See Horowitz' tweet below. In OUAT 3, fans got the Evil Queen and Wicked Witch as sisters. This year, OUAT presents its take on the Snow Queen, more popularly known as Disney's Queen Elsa in the "Frozen" movie.

Now, here is something for OUAT fans to ponder on. "Lost" alum Elizabeth Mitchell got a Once Upon A Time Season 4 role. Her character has connections to "Frozen's" Queen Elsa and Princess Anna. She is "possibly malevolent," reports Entertainment Weekly.

Meanwhile, Us Weekly gives Oncers fans a sneak peek into Queen Elsa's costume in Once Upon A Time Season 4. See the behind-the-scenes photos here.

"Useless Elsa costume fact- this outfit has 8 pieces to it. Perhaps one is tremendously bad at strip poker and is covering ones ass," Georgina Haig said on Twitter, getting nearly a thousand "retweets" and "favourites."

Still on Twitter, Ginnifer Goodwin has no BTS photo of Snow White yet. Her husband Josh Dallas, who plays her on-screen true love Prince Charming opts for a Granny's photo teaser. Fans await to see in Once Upon A Time Season 4 whether this power couple has a history with Queen Elsa in the still untold stories of the Enchanted Forest.

Jennifer Morrison has no specific OUAT 4 update yet, but she was recently in Australia for the OZ comic con. Colin O'Donoghue, who plays Emma's love Captain Hook/Killian Jones, is staying on the spoiler-free side, too.

Lana Parrilla, playing the Evil Queen Regina Mills, opts to look back to good times with her "sis," the OUAT 3 Wicked Witch actress Rebecca Mader in this Twitter post. Sean Maguire, playing Robin Hood, lets fans know he is back filming for Once Upon A Time Season 4. He is not revealing, however, how he would dump Regina now that his Marian (Christie Laing) is back - or would he?

Here's our first #OnceUponATime #titlespoiler of season 4 -- hope to see ya this fall! pic.twitter.com/2r13wOltK9

— Adam Horowitz (@AdamHorowitzLA) July 7, 2014

Horowitz also announced on Twitter the Once Upon A Time Season 4 premiere date is on Sept. 28, keeping it a Sunday night favourite on ABC.

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