I let my daughter stand next to me and push the floor pedal while I'm piecing. She understands that she needs to pay attention and lift up her foot when the needle is approaching a pin.
I also have let her arrange patches on the floor, like a design wall.
She loves going through my scraps and picking out stuff she likes.
I've even taken her to the sit n' sew at our LQS. She picked out some FQs, and I helped her to make a purse.
I have taught her the technical jargon of a sewing machine so she can identify the parts (presser foot, needle plate, foot pedal, etc.).
I'm starting my 1 year old on the jargon too. He now knows bobbin, needle, and thimble. We're getting there. :)
Good luck. Also, remember that at 4, their attention span isn't huge, so little jobs are better to do that ones that take a while.