DC Comics will be doing The Dark Knight IV, which will be written exclusively by Frank Miller.
Miller confirmed the news to Newsarama.
"I thoroughly applaud what [Brian's] doing," Miller said. "But now that he's doing his, it's now a four-part series. I'm doing the fourth."
The Dark Knight III: The Master Race comes out November 25th, which is actually written by Brian Azzarello with Miller basically on board as a consultant.
The article states Miller "indicated he was aware of Azzarello's story, and he emphasized that he supported it" and goes on to note Miller said that after he read The Dark Knight III in full, Miller intends to create his own fourth chapter. It's not known if Miller will both write and draw the new series (Andy Kubert is doing the art for DKIII).
The Dark Knight III was initially said to be the last of the DK chapters, but Miller now says it's a four-part series as the article notes:
"According to Miller's language that the series is now 'four-part,' the former 'trilogy' would presumably now end after the fourth volume."