This study investigates 4-6-year-olds’ ability to reason about prior and posterior probabilities, and how they update their decisions based on new evidence. Across two experiments, children made a prior probability guess and then, after receiving additional information, a posterior probability guess. Our findings suggest that children as young as four can make accurate prior probability guesses and in some cases, update them when given new evidence. Children’s ability for probability updating improves with age. These results suggest that the ability to reason about posterior probabilities emerges earlier than previously thought, by age 4.