Why has the stranger in the yellow suit come to the Fosters' house?
At the beginning of Chapter 4, we see the stranger arrive at the Fosters' house. His appearance may seem creepy to the reader, since he arrives at sunset, pauses there by the gate, and watches Winnie for a few minutes before talking to her. His unusual behavior, jerky movements that remind Winnie of a marionette, and black hat make us nervous about his intentions. But Winnie talks to him honestly, answering his question about how long her family has lived in that area.
Why is he there?
Winnie's grandma comes out, and the stranger makes chitchat, then tells her, "I thought you would probably know everyone who comes and goes," hinting that he's after some information or at least some gossip. He listens to the mysterious music with Winnie and her grandma, then walks away.
The chapter ends without the narrator saying why the stranger came to the Fosters' house. We have to figure that out by reading much further.
After we've met the Tuck family, we understand that the stranger was looking for the Tucks. We can tell because he was intrigued when Winnie said that her family had lived in that area "forever." He's figured out the Tuck secret and wants to snatch away their land so he can use the magic water for his own greedy purposes.