My 4-year-old nephew has been hitting other kids on the playground. How do I get him to stop?

If the hitting happens more than a few times, that child certainly is expressing his extreme emotions like anger towards the other kid. From this article: http://sg.theasianparent.com/how-to-handle-a-hitting-toddler-what-you-should-never-do/ I've learned that the best action to take when the situation arises is to take the child in a less distracting place. Telling him "No hitting" might not actually work but showing empathy will. We try to acknowledge the kid's feelings. "(We) Listen, hold the child and allow him to cry."

same on my toddler i just talk to him and told him that its not good to hurt others.