<human>:  What are the best books to buy for a 4-year-old?
<bot>: Some of the best books to buy for a 4-year-old include "Goodnight Moon" by Margaret Wise Brown, "The Very Hungry Caterpillar" by Eric Carle, "The Cat in the Hat" by Dr. Seuss, "Where the Wild Things Are" by Maurice Sendak, and "Harry the Dirty Dog" by Gene Zion.