what is the theme development of chapter 4?

Chapter Four reveals the object of Gatsby's yearning which has been apparent since the first chapter: it was Daisy, and his love for Daisy, that caused him to reach out toward the mysterious green light. The green light serves as a symbol for a number of things: among them are Gatsby's dauntless romantic optimism, Daisy herself, and the American dream.

Even Gatsby's infamous parties are thrown for the sole purpose of attracting Daisy's attention; she is his animating force. Everything Gatsby does and has done is out of love for her: he has reinvented himself as a cultured millionaire solely to court her approval. In this way, Daisy seems to serve as a symbol of the American Dream (at least in its 1920s manifestation); her corruption and emptiness will reveal the corruption that has befallen the great dream itself.