Dark Tides: Complete Series

I know this is technically a review on all six books in the series.. On what says is the completed 4 book series but I don't see any other options.
I really enjoyed the first 4/5 books. I hated the last book. I hated the ending. I wish I had stopped reading after book 4. I liked that ending. I was content... Did the author get bored and just wanted to quickly finish it up? Did they run out of time? WHAT HAPPENED HERE?!?!!?! gah! If/when I re read in the future I will only read the first four books in the series.