So the answer to both of those questions is kind of the same. I think I can recognise the mechanical superiority of 4A over AC4 but I really love the story AC4 tells, especially in the tranquil domesticity of the relationship between the Raven and Fiona and how it
contrasts with how criminal the mission objectives themselves are. With For Answer, I was under the impression that much of its story was neutered in translation so I may feel differently if I ever return to it in Japanese but I never found something to connect to in that game the same way.
I also thought the Arms Forts were gimmicky and unfun.