Looks like, as usual, some guy on a forum was right. The fourth Borderlands DLC is all about a Ninja Claptrap leading a robot revolution. It's called, helpfully, "Claptrap's New Robot Revolution."
It'll cost £6 and will have over twenty missions, it'll open up ten more skill points and three more backpack slots, and will be out this September. Here are the two screens they've released:
What's next? Alien Nazi vampirates?