I think book 4 was simply too long and he had so much stuff to wrap up that he decided 2 books was a better idea. He stated that we would all be happy with the speed with which the 5th book would come out. I am assuming he has a lot of it already written.
There are major factors going on. We have Gavin/Dazen's storyline, Teia, Kip, and Karris v Andross, Liv as a goddess, Ironfist, and that doesn't factor in the White King and The Dancer.
I read the first two chapters he posted up... I have been waiting for something new from him for so long I just had to. Where his story takes me, I am sure I will be satisfied with it. Once book 4 is over, I will have a moment where I get mad that I have to wait for a conclusion.