I would go with the sewing cards myself. It will help his eye/hand coordination. Many small kids do not have the fine motor skills needed to handle a regular needle and thread. As he becomes more adept, teach him to hand embroider simple shapes.
When i was taught to use a machine my sister drew shapes on paper and i had to be able to follow them using the unthreaded machine. not until i could follow the lines was I allowed to actually sew. When the time comes that he is ready to use a machine I would use one that has a speed control so he has to sew slow.
Personally I would be uncomfortable with a 4yo and a machine but I don't know your GS.