Season 4 is going to be very different - that book is basically a Western-inspired side story that almost entirely takes place on Ilus/New Terra. But they’re clearly adding more stuff in the Earth/Mars/Belt story between Avasarala, Bobbie, and Drummer/Ashford, so it might not feel that different.
Season 5 has already been greenlit and that book was the best of the first six, though, bar none.
The series isn’t indefinite, though - it was written as a trilogy of trilogies. That’s why season 3 felt like an ending - it was the end of the first trilogy. Book 9 will be the ending of the whole thing and it will be out within the next year, most likely (the book writers, who are in the writer’s room on the show and are executive producers, have been clear on the 9-book plan from the start and have only had two delays on their one-book-per-year schedule - book 8 was delayed a few months, and they decided to push book 9 back a bit as well to make sure they don’t rush the ending while also giving them time to work on season 5 of the show).
Bezos is also a huge fan, so it’s almost certain we’ll get through season 6 (end of the second trilogy) at minimum, and it’s more than likely we’ll get all 9 books adapted. Way back when he announced the Kindle, he debuted it with an image of the first book in the series’ cover on the screen, and that was way before the show was even a possibility.