Final answer:
The percentage of 4-year-olds who have friends cannot be determined, but it is recognized that children aged 2-4 years old start to display increased social behavior and enjoy playing with other children.
Step-by-step explanation:
Based on the provided information, it is not possible to determine the specific percentage of 4-year-olds who have friends. However, it is generally recognized that children from the age of 2-4 years old start to display an increase in social behavior, enjoy playing with other children, and can cooperate and share with their peers. Keep in mind that individual experiences may vary, and children develop social skills at different rates.