It was all a bit too real compared to previous seasons. Especially season 3 which was so driven by Piper's underwear business which was a very light storyline for the most part.
The majority of the conflict was external to the inmates. Sure we had the neo-nazis against everyone else (which was weirdly comedic) and Maria v Piper but neither of those plotlines were carrying the season. Ultimately it was the damning effect of budget cuts on quality of life and prison brutality, both of which are hard to watch when you know there's a point being made.
Similarly, Poussey's death and the aftermath had such clear allusions to events like Ferguson - of course that's going to be tougher to watch when you know that's been plenty of people's real lives/deaths.
I actually found the toughest bit of the whole season the last episode when the MCC are brainstorming ways to spin Poussey's death for the media - questioning whether she was violent, digging through her past. Partly because I work in PR, so I can imagine that happening exactly, but also the heartbreak of knowing that there are police forces who almost certainly had those conversations for weeks after similar incidents.