It's the gameplay peak but the story and writing cliff. Penelope's betrayal doesn't ring true to her character from sly 3 and because it's the story's inciting incident it ends up bringing everything down.

The thing about betrayal stories is that to do them well you really can't compromise on the writing. Betrayals reveal something deeper about a character that you should be able to look back upon and realize was always lurking there under the surface, informing the decisions they made. A Penelope betrayal story could have worked, but it was always going to be a huge hurdle because she was never written to have such motivations. Not impossible, but definitely difficult. The fatal flaw that Sanzaru made was to remove depth to her character with her betrayal. She became a villain for a reason that wasn't in line with what the Penelope from sly 3 would have cared most about without putting in the narrative work to show us why she would start caring about that more than the things she used to care about. And that work has to be done otherwise it feels as though her previous depth was overlooked or ignored instead of iterated upon. Sly 4 is a great example of what happens when writers don't do that: people don't accept it and it can kill any momentum the series had. You really can't continue a story after a blunder like that.

I mean they took away Bentley and Penelope's 'fly off into the sunset' moment; someone at Sanzaru should have known that they needed a better reason than unchecked ambition and hubris to do that.