I taught 3 of my grandchildren to sew on a machine, they were ages 9 and 7. They started out sewing on paper napkins. I made them all learn how to thread the machine correctly because I knew I wouldn't always be there to help them.
At 4, threading the machine might be a challenge, but if the child wants to learn something, he or she CAN master it eventually. :D
ETA: My son and I devised a finger guard so the kids wouldn't sew over their fingers. It was fairly simple, was done with copper wire.