Based on what I've read here, I think that's been tried and doesn't work. Until it does, that four year old gets to go out in the world with fear, with the adults around seeming to be unable or unwilling to help.
When my kids were younger, and had these very same troubles, I told them what I was doing that they might not have been able to see, and that they shouldn't start a fight but they had every right to finish one, and that I'd stand behind them if it came to that.
Just having the confidence that they could defend themselves in the moment helped a lot.