Even if you're example is a bit extreme, I am pretty sure that a 4 years old has not the maturity to understand the far-reaching consequences of such dire actio

Even if you're example is a bit extreme, I am pretty sure that a 4 years old has not the maturity to understand the far-reaching consequences of such dire actions. Understand that it is bad yes, understand that it is definitive no. This is also why depending on the jurisdiction and their age, minor are judged according to special laws. And I am not sure a place exists where a 4 years old would be put on trial.