To say that Chapter 4 represents a turning point in the novel means that it marks some significant change in the plot or narrative. Nick begins this chapter by listing the usual guests at Gatsby's parties, and Gatsby and Nick spend some time alone together. Gatsby tells his ludicrously romantic story, which Nick can scarcely believe until Gatsby produces apparent evidence of this history: a photograph. Nick and Gatsby go into the city, and Gatsby introduces Nick to Meyer Wolfsheim. Later, Jordan explains the history between Gatsby and Daisy and how Daisy ended up marrying Tom Buchanan.
This chapter, then, really initiates the present story line between Gatsby and Daisy and sets into motion all the events which follow. The chapter also begins to confirm some of our—and Nick's—suspicions about Gatsby's illegal activities, given the crowd he runs with and his sudden fortune. In other words, Gatsby becomes both more sympathetic and morally problematic in this chapter. Perhaps Nick "chooses" this time in the story because he began to become aware of Gatsby's complexity at this time, too. It is also possible that Nick problematizes Gatsby here so readers can understand the man better as we continue to read his story. Nick doesn't want us to think Gatsby is a saint, but compared to most of the other characters in the story, he is somehow "great," and this sets him apart.