If one reads "A Rose for Emily" as an allegory in which Emily represents the Old South, then the interpretation of Section IV involves the desperate attempts of the South to retain a defeated lifestyle. Such passages as
Then we knew hat this was to be expected too; as if that quality of her father which had thwarted her woman's life so many times had been too virulent and too fuious to die.
Thus she passed from generation to generation--dear inescapable, impervious, tranquil, and perverse.
She died in one of the downstairs rooms,...on pillow yello and moldy with age and lack of sunlight.
indicate the erosion of a way of life in the wake of new generations and the Northern destruction.
Section four in particular talks about the passage of time after she buys the arsenic. We don't learn until the end what she intended to do with it, but looking back we realize that she killed Homer, kept his dead body in her upper room, and went on with her day to day existence. Section four provides a list of the kinds of things that happened in the intervening years up until her death. She gave china-painting lessons for a few years, but stopped as a newer generation of children came along who were no longer interested. The town modernized its postal system, but she refused to let them put up the house numbers. Both details show how Miss Emily remained stuck in the past while the town evolved around her. The narrator tells us that she became a duty and a care, always impervious, and ultimately perverse. Section four gives us the final details of her life as it sets up the shocking reveal in section five.
I agree with the above post. For so much of the story, Emily is a passive character. Other people do things which impact her, but she does little to shape her own life until this section of the story. Of course, nothing she does is healthy or helps her become either happy or content.
Emily has spent a whole lot of her life being victimized by a father who isolated her and drove away her suitors. She was victimized by her father's death and even by the cousins who came to "talk sense" into her. I would say section IV is when Emily finally decided to stop being the victim. The man she loved seemed to be drifting away, and so she put a stop to it ensuring he would never leave.