Chapter 4 ends with Scout revealing that she heard laughter coming from Boo Radley's house on the day she rolled into the yard. Why might Harper Lee have chosen to save this detail for the end of the chapter, instead of sharing it right after the incident occurs in the book?

There are several reasons Harper Lee may have chosen to include the laughter Scout hears from the Radley house at the end of the chapter instead of in chronological order.
First, the scene is chaotic; many things are happening at once. By including this small detail at the end, Harper Lee ensures that it will be noticed.
Second, she may have wanted to emphasize the detail. By stating it at the very end of the chapter, it becomes a prominent detail rather than a small one.
Third, when Scout is in the Radleys' yard, she is traumatized from fear. Only later, when she is safe and back at her home, can she review the events in her mind. This would naturally happen at a different time.