R. Scott Bakker has provided an update on his forthcoming books. He has confirmed that the final book in the Aspect-Emperor series has been split, as was anticipated from Overlook Press's schedules a few weeks ago.
The first of the two books, The Great Ordeal, will be published in July 2016. The second, The Unholy Consult, will be published at some point in 2017. Apparently the expansion of the series from three to four volumes necessitated a redrawing of the some of the contracts.
In previous comments, Bakker confirmed that his intention is still to write a further duology in the world but that, at a push, the series can end with The Unholy Consult.