While learning to count, all children will skip numbers. The opening dialogue illustrates this process: "1, 2, 3, 5." "Where's 4?" "4 went on vacation." With 4 missing, 1, 2, 3, and 5 all speculate as to where 4 is vacationing and what 4 is doing on his vacation. These fantasies range from going to the moon, swimming to the bottom of the Marianas Trench, and running in the Olympics. After 4 returns, 4 tells them all about his camping trip to 4 Rainbow National Park. 4 and his family went hiking, swimming, canoeing, and bicycling before coming home to Numberland. The book's narration is written in a manner that entertains both the child and the adult reading the story, combining both learning and childhood imagination.