List of season four episodes from Megan McCarthy. Looks very promising See also Steffan Andrews's list and the full hashtag list. Also this tweet and this tweet which aren't in the list.

01 & 02: Wingin' It

03: Now You Sea Them…

04: Meet Princess Skyla

05: The Student Becomes the Teacher: The Return of Sunset Shimmer

06: Luna-tic

07: Pinkie Apple Pie - Pinkie Pie becomes the Princess of Pie (mini-arc part 1)

08: Don’t You Dare - Rainbow Dash threatens to leave the Daring Do fandom when they turn her hero into an Earth Pony.

09: It Ain't Easy Being Cheesy - Justin Bieber guest stars in our musical episode. (Continues in 16)

10: Oops - Rarity is the star of a play but is replaced by AJ.Twi forgets and tells everypony RY is the star. Ponies are pissed.

11: Bats - It's just like the musical "Cats", but with, you know, bats. And ponies. But mostly bats.

12: The Return of the Return of Harmony - Discord is back! And his new personality is kind of meh. Stupid writers

13: Wrap It Up - Every loose end is tied up. Hilarity ensues when the ponies realize there are 13 more episodes to go.

14: Pie in the Sky - Pinkie Pie becomes the Princess of Pie (mini-arc part 2)

15: Who Are You? - David Tennant guest stars as a random, nameless background pony no one has ever noticed before.

16: It Ain't Easy Being Breezies - (Continuation of 09) ends with Justin singing with the Breezies

17: Just Forget It - The CMCs try literally everything they can think of to get their Cutie Marks. They don't. The end.

18: Death Comes to Ponyville - Didn't bother to read it. Was super busy that week. I'm sure it will be fine.

19: Maude Pie - Pinkie Pie becomes the Princess of Pie (mini-arc part 3)

20: For Whom The Sweetie Bell Tolls - cuz literary and makes us sound smart and stuff and,dude, is anybody else hungry?

21: It Ain't Easy Bein' Weezy - a hilarious parody of The Jeffersons. Look, they can't all be gold, people.

22: Best Pony - New pony comes to town and she’s cool and likes craft beer and Legos and has a typewriter Cutie Mark

23: Spike and Rarity Sittin’ in a Tree - Nothing happens. That would just make things weird between them. Plus, his ex is totally psycho.

24: Let the Games Begin - Shining Armor and Cadance join the professional wife-throwing circuit. Destroy the competition.

25 & 26: Princess Discord - Discord becomes a draconequalicorn and replaces Celestia. As she always intended.

Man, compiling Twitter stuff is hard. So much information...it's a good thing my OCD can bow to the sense of something not being worth the immense amount of time to do it, otherwise I would never even get a chance to procrastinate on my homework.