Final answer:
Jerome Kagan's research categorized infants by temperamental types but the exact percentage of 4-month-olds identified as shy in Kagan's study is not provided in the resources. Therefore, an accurate answer cannot be stated.
Step-by-step explanation:
Jerome Kagan's research into temperamental categories among infants identified that a certain percentage of 4-month-old infants exhibit shy behavior. Kagan's seminal work in the field of developmental psychology focused on identifying the biological bases of behaviors like shyness in childhood and sought to categorize infants into different temperamental types based on their reactions to novel stimuli. Unfortunately, without the specific percentage given in his research within the resources provided, we cannot accurately state what percentage of 4-month-old infants are shy according to Kagan's studies.