We knew it was going to be four seasons either when the first one came out or when the second one came out (currently season 3 is airing).
They might have posted a teaser, but the existence of a fourth season has been known for years.
For context, the author of the books is working with the show. The books were okay, but not very good. Good idea, sloppy execution. So the author is going back and changing things. People complain about shows and movies changing things. In this case it's the author doing it. He's with every change because he's responsible for every change.
When season 1 ended and they didn't finish the first book, it was rumoured the series would go for six seasons, with two seasons dedicated to each book. The problem with that theory is that book 2 does not have any (or many) characters from book 1. Juliette isn't in it. So we figured they would mix the second and third books, as the third book continues the story from the first book as well as the second book, and has those characters interacting.
After the second season, we learned that the end of the first book would be in the third season, and that book 2 was going to be adapted. After the first episode of season 3, it was clear that book 3 was pretty much being thrown in the trash. And, possibly rightly so. Too much has changed from the first two. So in the second book, everything happening in the "before" times, wasn't in the book. They explained the Silos, they explained what they were for and why they were made, then Something Happens and they all go in and seal them. The book is called "Shift" because the characters cryogenically sleep for years and come out in "shifts" to work. So yes, minor spoiler, Donald Keene (renamed to Daniel Keene so as not to seem like a reference to the American president, who also has the given name Donald) does exist in Juliette's time, despite being from our time, some 300 years before Juliette was born. Who is he now? What is he doing? I have no idea, they changed so much.
I still think it's worth it to go back and read the books. Some call them novellas because they're so short. You can also listen to them in audiobook format. The first one is 15 hours, second one is 14.5 hours, third one is 10.5 hours. For reference, the Dungeon Crawler Carl audiobooks are 8-10 hours each. The third one is right in line with that, and the first two are about half again as long. They're still considered short by audiobook standards — I expect to sink 25-30 hours into a book. IT by Stephen King was especially long at around 45 hours, but it was a thick book.