Di Bonaventura states that while the film does have a whole new cast, and none of the human characters from the previous movies, it is a continuation. A continuation of an existing story line, in an existing story universe, just with fresh faces and a new story.
The Hollywood News]
See the full excerpt below:
It’s definitely not a reboot. It’s an interesting question about what you should call it. On a certain level it’s a continuation of the previous stories, in the fact that it acknowledges what has transpired before it. It acknowledges in the last movie, the destruction of Chicago, it’s actually something that carries through the sort of emotional repercussions of that, not unlike 9/11 has emotional repercussions in the real world. In a fantasy world there are repercussions to what occurred.
That plays into the movie, moving forwards with a totally different human cast, who doesn’t know anything about the other humans, it’s not a reboot, but a continuation, yet you’re continuing with a new cast and group of characters. It was a big decision to do that.
We miss our friends that we did the first three with, and they were great, and they probably could’ve done more. But the advantage of doing it this way is that it feels almost like a first movie. It’s a very different dynamic than I’ve seen in a movie, I’m very curious. I guess Star Wars did that a little bit, but not so close together, the way we’re doing it.’