From ChevronOne :
Episode 18: The Stable Boy ~ Here we find out why Regina hates Snow White as mentioned previously.
Episode 19: The Return ~ Here we get more of a back story of Rumpelstiltskin and his son as mentioned previously.
Episode 20: The Stranger ~ Obviously this is the episode where we probably find out who "The Stranger" is.
From Entertainment Weekly:
Episode 21: Apple Red as Blood ~ According to the article's writer Sandra Gonzalez:
...I can tell you that it’s part of a final arc that will focus heavily on the major players rather than stand-alone stories.
Josh Dallas ~ All I can say is I just got the script for episode 21 and it is going to go someplace that the audience will not expect it ever to go