I don't know if I can answer this with certainty. When Jean was asked about her fourth book The Plains of Passage. She said it was the hardest to write because of the transition she was making in the story. So.... I assume the fifth book was planned. But maybe not ...I only read the first three books in the series, so I don't remember - was the fifth book always planned to complete the narrative? Or was it added on after readers had assumed the series was finished?