Without McAvoy maybe less than 4, he definitely made it a lot better. Him in another Split movie would have been a better decision (not that it needed a sequel etc.).
Think going into it I would have cared less about the Split Villian. Mr. Glass was much more interesting to me. But it seems that got turned around in this movie.
Yeah, it wasn't SLJ's greatest performance, but that blame falls on the script. Spoiler As he was twitching, I was thinking "Really...that's how you're using this mother f'er"