The four little elms have great symbolic value to Esperanza. They represent survival, resilience, and, appropriately enough given the literal Spanish meaning of her name, hope. The trees act as an inspiration to Esperanza, holding out the chance of a better life than the one she currently lives.
Although the trees were planted in such unpromising ground, they've nonetheless managed to grow, rising above the concrete, the asphalt, the car fumes, and all the other unpleasant features of life in this neighborhood. These four little elms have defied the odds in growing amidst such a challenging environment, and Esperanza hopes that she'll be able to do the exact same thing. Mango Street may not be the best neighborhood, it may be beset by all manner of seemingly intractable social problems, but it's still possible to experience moral, spiritual, and intellectual growth in this part of the world.
Over the course of the story, Esperanza defies expectations, including her own, in realizing that Mango Street is a core part of her identity. She may well leave one day, but even if she does Mango Street will never leave her. Mango Street has made Esperanza, just as it has made the four little elm trees that the city authorities planted by the curb.