The example that best illustrates Twain's use of satire in chapters 4 and 5 is the battle for custody of Huck.
In these chapters, Twain uses satire to critique the notion of civilized society and its flawed institutions, such as the legal system. The battle for custody of Huck between the Widow Douglas and Pap represents a satirical portrayal of the legal process and the absurdity of a court deciding who should have control over a young boy.
Twain satirizes the idea that the Widow Douglas, who represents the civilized society with her attempts to "civilize" Huck, is deemed a more suitable guardian than Pap, despite her lack of genuine care and understanding for Huck's needs. The court's decision to award custody to the Widow Douglas is portrayed as a mockery of justice, as it disregards the individuality and desires of the child involved.
Through this satirical portrayal, Twain highlights the hypocrisy and shortcomings of a society that claims to uphold morality and righteousness, yet fails to recognize the true well-being of a child. The battle for custody serves as a vehicle for Twain's criticism of societal norms and values, exposing the absurdity and injustices within the legal system.
Therefore, the battle for custody of Huck best illustrates Twain's use of satire in chapters 4 and 5, as it satirizes the flawed legal system and the misguided priorities of society.