I watched season 4 of The Expanse this weekend, which I'd been anticipating highly. I'll try to summarize my thoughts without getting into specifics. I know some people's definition of "spoiler" is a lot more severe than others. If you're an extremist, you should probably stop reading this now.
First, a comment on the changes to the show now that it's on Amazon: We were told that being off basic cable meant that they didn't need to censor the content, or fit inside a 60 minutes-minus-commercials time frame for each episode. That said, the episodes were still pretty much 45-50 minutes each, and not censoring means they just said "fuck" a lot more. No real huge departures here in terms of format. Also of note: The weird kind of cheap-looking hydroponic thing in the Roci's galley has been replaced by something that looks less like a flimsy prop with plastic plants and more like something functional. I guess Amazon let them spend a few extra bucks :P
The season is broken down into four plot-lines: The Roci crew on Ilus/New Terra, Bobbie on Mars, Ashford and Drummer in the Belt, and Avasarala on Earth. Aside from the first one, none of these are really in book four. Bobbie's story was her side adventure from Gods of Risk, but greatly expanded, but the others were pretty much original material. On the whole, I thought those three side-plots were kind of dull. Having read the whole series to date and knowing what's going to happen, it's clear they're designed to set stuff up for the events of season 5. Besides moving pieces on the board and laying some groundwork for the future, they didn't bring much to the table. They'll pay off next season, but I would have liked a little less in favor of more of the main story.
I thought the main plotline on Ilus was done very well. They could have taken their time on it a bit more, but I was happy overall. The look of the planet, the settlement, and the alien structures was very close to the way I'd pictured it when reading. Almost uncannily so. The only place where it fell a little flat was at the big climax, which seemed like it was down-scaled for no other reason than to contain the CGI costs. Disappointing, but forgivable.
Overall, I liked it. Not as much as previous seasons, but the book it's based on is kind of an outlier in the series and was probably tricky to adapt. After three seasons with an ensemble cast jumping all around the solar system, this season's main action took place with a small group of characters telling a small story in a single location. It didn't have the same energy or intensity as previous seasons, but neither did the source material. I expect season 5 will get us back to The Expanse we're more familiar with, and holy shit it's going to blow people's minds.