The first chapter proper is the Book of Air that gives the overview of the setting, history, cosmology and social mores and customs. The history in particular is nicely done in broad strokes making it seem far less jarring and more complete than the equivalent section in third edition.
It is also obvious that they did try to make this edition as timeline neutral as possible, but unsuprisingly that proved to be impossible to do completely and understandably the writers defaulted to the current timeline when that happened. I will discuss this further in some specific cases in the remaining parts of this review.
For those familiar with the setting there isn't much to add. While I like Rokugan a lot, but it is a standard fantasy setting with a coat of Japanese laquer. This isn't necessarily bad but the exoticism might become disparaging at times, in particular when the trivial is seen as essential, and when the different elements that are pastiched together are not carefully integrated.
Also of note, in regard to previous editions, is the end of the "Rights of the Challenged" or rather the fact that the challenged party no longer decides the method of the duel. It was something I felt didn't mesh with the way Rokugani are described to be obsessed with Honor and if you look at societies or periods with shame cultures and duelling as the main method of settling social disputes it didn't made sense either.