Technically speaking, it would be six episodes, because "Curse of the Kobra" is also two parts. However, we've always counted multi-part stories as one episode, so that's why I said "four episodes."
Due to audio problems my suggestion had to be edited out, but it was Ellen Page. In Juno we saw he play the smart, spunky title character, and I think she'd be okay in the role of Harley Quinn. I could also see Anna Paquin in the part.