He's back! This will be interesting to see The Lich's return. And ever since I read the episodes for Season 4, I have a theory of who will be involved with the big battle at the end of the episode:

1. Finn.
2. Jake.
3. The Lich.

And here are possible characters that might be involved with the episode:

1. PB.
2. FP.
3. Marceline.
4. Ice King.
5. Billy.
6. The Snail (since there will be a Season 5).
7. The Bear (from "In your Footsteps").

Well, we will find out if this production is correct.

I also think that they repeat the Incendium and Hot to the Touch two parter idea to seperate the final episode of Seaon 4 and the first episode of Season 5.