From reading the OP post they’ve done everything but getting the police involved, which clearly shouldn’t be a step for a 4 year old…... They’ve talked to the directors and teachers.

I ask again, what’s the solution?

You’d be surprised how well kids know the difference between right and wrong.

You’re not meeting violence with violence…. Using self defence is not violence, why are you continuing to go down this path? And why are you saying the self defence is violence? Who’s told you that? Because that’s bull shit. That’s reasoning people use to bully people since they “can’t” defend themselves. You’re enabling this shit if you continue to choose to think self defence is violence. It’s not.

Kids need to learn from an early age that bullying is not acceptable. Getting that lesson straight from the bullied kid is the most effective method.

In short, don't start shit, won't be shit.