Is there something going on in her home? Big changes causing the parents stress? Because children pick up on stress like that. Are they talking to her about school coming up by any chance and freaking her out? There is usually a reason for a behaviour change like you're describing if she used to be sweet. We adults have to remember to think like a 4 year old in a 4 year old world to figure out the root of their problem. We're so used to rationalizing everything on an intelligent level, but for a child it's basic problems, eat, sleep, poop, happy, tired, sad, YKWIM?

Good luck! This child may even be stressed about your wedding if she senses big changes are coming in your life and can't figure out in her little brain how to process the changes. Just a few things that crossed my brain.