She barely appears in this season. But maybe it's like you said, there are no memorable things happening. The only good thing about Piper in this season is her continued descent into being a terrible person. However, I still feel as if it just went at an awkward speed this season. She was a self-centered ass, but out of the blue, she has basically became a wannabe gangster who hates everyone. That development went way, way too quickly. Outside of that general character arc, her life was beyond boring. It may have to do with the fact that this is based on a book. I am guessing they followed the book but then had to instantly jump away from it when it did not provide enough for three seasons.
Stella, I don't understand the need for her. Alex was already breaking down which caused tension. No need to throw in a random person who's sole purpose was probably to just get the audience to say, "Shit, a really hot character with a sexy accent." Piper is shallow, but she also loved Alex beyond her physical looks. The wildness, the chaos, the excitement. Stella brings nothing to the table other than looks and a few good ideas for the panty business.
The development of Caputo was excellent! I especially enjoyed his total collapse at the end when he abandons his core values after so much struggle. His entire life has been him standing for his values and it has gotten him nowhere. Finally, he breaks and he abandons them after continued struggle. Good choice overall. The only real problem was his almost total lack of empathy when his people left in the final episode. Even after he breaks, he is still Caputo and he was bound to show a few more feelings than we got.
In one way, I LOVED this season. May even be the best one for me. But god, the flaws are there and they are huge. I was engrossed in some of the drama. Some flashbacks were top-notch, certain characters really became fleshed out, and some people just made you want to hate and love them. The flaws, though, are large. Piper's story has basically collapsed on itself. Alex became boring and annoying as it was literally just the same thing with her. Way too many useless flashbacks. Odd and stupid stories like Crazy-Eyes' writings, Stella and Piper, and the Norma stuff made parts of the show repulsive. I best end this as I have just written an essay.