In terms of structure, Chapter 4 encompasses the "B-Story" of the first story in Book 5. You can almost think of it as a sidequest. One of the things I like about it is that it returns us to a familiar place with a few familiar faces, too (though under very different circumstances)!
As you've probably come to expect from my writing, I always prefer to re-introduce story elements vs introducing completely new ones: it keeps the memory "footprint" low + readers love throwbacks. Or at least I know I do!
This chapter also builds upon the branch made in Chapter 3, so you may want to play it twice!