Splashee

@CypherHoof You mean, like the opening/ending book of FIM? Or the book Twilight Sparkle sold in the episode Fame and Misfortune? (yes, there are two books!)

Sorta both? Sunny is seen reading a book in at least one of the teasers, and she has cutie mark pins. Maybe its like a scout thing, or maybe she is just a fan of FiM, which isn't taken seriously by most "adult" ponies?

It would make sense that the opening/closing book actually has ALL of FiM in it, so basically the legend of the two sisters, the final song and montage, and everything that happened in between?

(seem to recall that Sophia the First used a similar "going into the book" motif in the pilot)

I also notice that in the teasers they don't have cutie marks, but in the accidentally posted bedding merch, they do?

You are right. it makes sense to be the main story book for G4. It is just scary to touch the G4 story after closing the book, so soon.

To me, I think all the main stuff will be dealt with in the Movie. If they don't have cutie marks in the beginning, they will definitely have it at the end of the movie (Hasbro has to sell MLP toys with cutie marks to make them MLP toys in the first place). Also, pretty sure that 3D quality is from the Movie.

I have heard from most fans that closing the book at the end of G4 was the right thing to do. Opening it too soon might be a bad thing. So, to be on the safe side, I hope it is the friendship book she finds instead, with the actual Mane 6's lessons inside.