@worshipguitar I'm not saying that I agree with the teachers wording, but I think it is reasonable to ask if you are sure this
is the first time, or is this simply the first time the teacher has got
you involved.
If the teacher has raised this just with your child on a few occasions already, then that would certain put this in a rather different light. Your child says they didn't know, a 4-year-old is both capable of completely forgetting, but also of lying - of claiming that they didn't know when they did (and, maybe, are embarassed to admit that they had forgotten.)
I repeat, I am not saying that this
is what has happened, but suggesting it as a possible alternative.