Volume 4 of Ōoku: The Inner Chambers is a transitional work, covering the end of the reign of Iemitsu, zooming rather rapidly through the thirty year reign of her successor Ietsuna, and then getting stuck in to the career of Tsunayoshi and the arrival at her court of the young nobleman Emonnosuke. It is also about the historical legacy of Arikoto, a central figure of the previous two volumes, whose valet ends up as Tsunayoshi's father.
The background to the story is of course a Japan which has lost 75%-80% of its men; but I feel the plot is more and more about the exercise of power, Iemitsu's wise and enlightened decisions - including "coming out" as a woman ruler and permitting other lords to do the same - contrasted with Ietsuna's indolence. We then see Tsunayoshi as largely concerned with using power for her own sexual pleasure, to the annoyance of her courtiers, and the end of the book suggests that Emmonosuke's arrival will take her and her rule in a new direction, though we cannot be quite sure what. It's enough to make me want to get the next volume anyway.