What is a summary of Chapters 4 and 5?
You can find very detailed summaries of those chapters (as well as each chapter of the novel) here, but as I understand it, you're looking for brief summaries: a way to understand just the main events of each chapter. I'll provide those below. And, if you need to summarize both Chapters 4 and 5 in one sentence, you can say "Jess and Leslie become good friends, create their secret world of Terabithia, and get back at the bully Janice together."
In Chapter 4, Leslie enjoys winning races at school, and Jess enjoys art class with Miss Edmunds. Leslie used to go to a wealthy school and is having a hard time adjusting to her new life in the country. Her class finds out that Leslie likes scuba diving and doesn't have a television, so they think she's weird. When Jess rescues Leslie from some bullies, they decide to play together after school. That's when they create their imaginary world of Terabithia together. They become good friends. Although their classmates and families don't quite understand Jess and Leslie's friendship, it continues to grow.
In Chapter 5, when the bully Janice steals Jess's little sister's Twinkies, he and Leslie take revenge on Janice by making her look silly to her crush and to the whole school. They do this by faking a letter to Janice from Willard, a very cute boy, asking her to meet him after school. Of course, she waits for him only to be disappointed. Then she looks foolish and feels very angry. Although Jess's sister is pleased with this revenge, Jess feels that perhaps they shouldn't have been so cruel to Janice.