Ask your kid to teach the younger kid

Often my almost 4 year old doesn't want to include younger kids in his play (or there is an older kid that doesn't want to include him). I find that if you ask the older one to teach whatever they are doing to the younger one, they are more willing to include them. Never force it, just say "hey will you teach him how to do that? He doesn't know how." It also helps them to feel more capable and empowered. A win-win.