Zap2it has published the title and air date of season 4 episode 9 to air on January 11, 2014: “Pinkie Apple Pie.”
Thanks to Jordan for pointing out that this is likely to be the episode that was previewed at San Diego Comic-Con—written by Natasha Levinger and storyboarded by Emmett Hall and Tony Cliff. Hence, we can look forward to to the song Apples to the Core!
Pinkie Pie becomes the Princess of Pie in the season mini-arc of 407 "Pinkie Apple Pie", 414 "Pie in the Sky", and 419 "Maude Pie"
— Meghan McCarthy (@MMeghanMcCarthy) April 1, 2013