Season 4 is mostly based on the fourth book, Cibola Burn, with some pieces of the fifth book, Nemesis Games.
The Expanse Season 4 just dropped all 10 episodes on Amazon — it actually had a surprise drop Thursday night, ahead of the announced December 13 premiere.
The Expanse is based on the novels by James S. A. Corey and eight books have been released so far, with the ninth coming in 2020.