Actually, it isn't. There is no individual class stories here. It's just another planet with another planetary story arc. Your class story will remain at Chapter 3, even though they're referring to the story content in this "expansion" as Chapter 4.
Not sure if it makes any difference to you now, but that is not accurate. The expansion contains continuation of the individual story lines for each class. http://www.ign.com/articles/2013/03/...y-far-far-away