Best book I've read this year. Best book I've read in a long time. Even better than the first three books in the series. And that was a pretty high bar.
I don't want to say much about the plot because I don't want to spoil it. Suffice it to say, it's epic. Not just in length (over 700 pages) but in scope. There is action across and beyond the galaxy. The themes are highly relevant and yet handled in a creative way. Nothing feels derivative though there are certainly a few intentional references. (While not as much as in the earlier books, some classic poetry is still quoted.) The characters are interesting and reasonably well developed despite the fact that they clearly exist to serve the plot and the creation of the universe. I'd say that exploration of this universe and a few key themes are the main points of the book.
There are enough references to earlier books in the series that I think The Rise of Endymion would lose something read standalone. It would probably still be a 5 star read, but it wouldn't be as good as if you'd read the other three books first. Largely, this is because some characters return and do not get much introduction because it's assumed you already know who they are.
The book doesn't answer every question it raises. I got the distinct sense that certain stories were left untold on purpose. Whether that was to stimulate imagination or to leave room for more stories in the series, I can't say. But I will say that the ending was almost perfect and it would almost feel a shame to mar that with a book #5 (though I suppose that book wouldn't have to follow chronologically).