One of the key themes of the entire play is the way in which humans make decisions and the fragile nature of the systems of law that try to uphold justice. The action at the very end of Scene 4 is important because it shows that Grusha is going to have to fight for the right to look after Michael in front of a judge as he is seized by soldiers sent from the city after Michael's parents began to search for him once more because they realised that they could only claim their property and wealth if they had Michael. This action is therefore incredibly important as it means that Grusha will have to deal first hand with a system of law that the play, up until this stage, has presented as being very precarious at best. Note how The Singer at the end of this Scene draws attention to this theme:
Who will try the case, on whom will the child be bestowed?
Who will be the Judge? A good one, a bad one?
The city is described as "the dangerous place," as The Singer recognises implicitly that where people are there will be danger, and the quote above makes it clear that what is known as "justice" is not an objective quality that is supported by the judge, but rather a subjective reality that actually has little to do with who is "right" or "wrong" but the judge who tries the case. Scene 4 is therefore important in drawing attention to the fragile nature of justice and also raising tension as the audience begins to wonder if Grusha will be able to keep the child that she loves.