When we say protagonist, we normally mean just one person – the hero or heroine of the whole shebang. But in this novel, we think it's totally fitting to have four protagonists. After all, the four individuals who pick Singer as their confidante are treated pretty much equally in the narrative. They all have a roughly equal number of chapters devoted to them, they all grow and change during the course of the novel (character development is a good thing for a protagonist), and we're definitely rooting for all of them.
Having said that if we had to pick just one protagonist, we'd argue for Mick Kelly. Mick has a ton in common with her creator, Carson McCullers, and her story often takes center stage in the ensemble.