During their Q3 Investor Call, Hasbro announced that they are working with AllSpark Pictures to create a feature length My Little Pony movie that's slated for release in 2017.
Joe Ballarini (Ice Age: Continental Drift) is on scriptwriting duty for this movie, while Meghan McCarthy (Party of One, Lesson Zero, Canterlot Wedding, Twilight's Kingdom) has taken on the role of co-producer.
You can read up on the full announcement here.
UPDATE: A tweet by Meghan seems to imply this is indeed a gen 4 movie with the Mane 6.