It takes practice to learn how to be a good friend ― practice at being kind, supportive, trustworthy, and a good listener. For a four-year-old, a friend may be a regular playmate or a child they just met at the park. At this age, they can engage in cooperative play, but they are still developing the ability to understand other perspectives. They may assume that their friends think the same way they do ― and can get upset when a friend wants to play a different way. They may also need help introducing themselves or welcoming others into their social circle.