Much happens in chapters 3 and 4 in a Tale of Two Cities. Lucie sees the evil in Madam Defarge, and watches her knit, she asks her for leniance, but Madam Defarge calmly tells her that she has none. This foreshadows what is to come later in the novel. Dr. Manette goes after Charles and sends a note saying that he is okay, Dr. Manette returns four days later without Charles, but assures Lucie he is okay, but still in prison.
Dr. Manette uses his position as a former prisoner to become a Dr. inside the prison, that way he can keep an eye on Charles. He realizes what his status as a previous prisoner has afforded him: the ability to do whatever he likes inside the prison with no suspicion.
Everything in chapter 3 and 4 is a waiting game. No one knows exactly what will happen, what their actions will cause.