For them, it was a business decision. The Expanse cost over 10+ million per episode to make, and people weren't watching it at the time it was broadcast. So, it got bad viewing numbers, because of the DVRs and similar things. SyFy unlike Amazon Prime isn't made from money.I find it crazy how SyFy finally did something right again -- the last time being the Battlestar Galactica reboot -- and they go and cancel it... They can be as bad as Fox sometimes... no, make that most the time.
https://arstechnica.com/gaming/2019/07/weve-got-our-first-peek-at-s4-of-the-expanse-and-an-airdate-december-13/Details about this long-awaited fourth season have been scant, although we know that the first episode is titled "New Terra." It's supposedly based on Cibola Burn, the fourth novel in the series. The crew members aboard the Rocinante travel to Ilus, a colonial world where a dispute has broken out between Royal Charter Energy (RCE)—the corporation that claimed rights to the world—and the first settlers there. There are also surviving members of an alien civilization living there, who created the protomolecule that featured so heavily in the story arc of season three in particular.
This first teaser doesn't add much information. We see a strange new desert world, the crew gearing up for a skirmish with REC security forces, and it looks like we're in for some pretty exciting space battles and a certain number of casualties, judging by a haunting shot of bodies floating in the expanse of space. The final scene shows Royal Charter Energy security chief Adolphus Murtry (Burn Gorman), pointing his gun at an unseen foe: "I hope that answers your question." I have many unanswered questions, but I'm happy to wait a few months for answers.