I started watching the show after the season 3 finale, so I had everything on Youtube and didn't have to wait for new episodes. But this wait for season 4 is soo long! I hope that it will be worth it.
My speculations are that Twilight will continue to stay at the library with just a couple of Royal Guards. Celestia may send her some powerful magic stuff for her to examine. I saw how other people here said that she might teach others about the magic of friendship, because Celestia said that they are all her students now. I don't think I agree with that. Celestia might not have meant "students" as Twilight is, or was, to Celestia, but "Twilight's students" as in Twilight being a role model for them.
Changelings would be good for the season 4 finale.
Just my speculations, nothing confirmed.