Update: Tiny Tina's Assault on Dragon Keep has been confirmed as the title of the fourth DLC. It will be out on 25th June and cost $10 or 800 MS Points, though it's included in a season pass.
Elsewhere Gearbox PR man Steve Gibson noted that "I did very much enjoy my nap!"
Original story: Take it from me, if you're going to sleep at work, don't do it when there's someone around. Also, don't do it in front of an advertisement for your yet-to-be-revealed DLC.
This is exactly what happened when a Borderlands 2 writer tweeted a pic of his lovely coworker snoozing away "like a lil angel," not realising that behind said sleeping fellow, laid a display for something called Tiny Tina's Assault on Dragon Keep. The tweet has since been deleted, but we have a picture of it from before that via StealthyBox.
Ostensibly this is the fourth DLC for Borderlands 2 that Gearbox teased at PAX East where it showed a trailer depicting Tiny Tina playing a tabletop RPG entitled Bunkers & Badasses.
This fourth piece of DLC - the final one for the season pass - is due in June, so expect more details on it shortly. In the meantime, we're trying to confirm that this is in fact the final title of the upcoming DLC.