A four year old's attentions span isn't very long, about 15 min. if I remember right. Give him a threaded needle and some fabric squares with the seam line drawn. He'll be busy and interested in spurts. When my grand would want to help me sew and do something different every 20 mins or so I sent her to help grandpa for a while. They just want to do what you are doing no matter what it is.