your 4-year old will not even be able to open the balcony door, they are very heavy. You can lock them. Is your son in the habit of getting up in the middle of the night to go exploring?? Close the drapes, they will make noise if he tries to move them.They are pretty heavy also, he would have a hard time moving them. Put something in front of the balcony door at night to block it. Keep and eye on him during the day, he would have to push a chair or table over to the edge of the balcony, climb up onto the chair or table to be able to just see over the balcony.