Gearbox has come forward and announced that they do not have any plans for a second season pass despite the announcement of more DLC for Borderlands 2. Gearbox told Polygon today that “the company has “been clear from the beginning” that the Borderlands 2 Season Pass would only include four add-on campaigns. There are no current plans for a second season pass.”
If you purchased the season pass, it seems that you’ll only be getting access to four of the five DLC packs.