I am not 100% sure but I believe the legal age is 14 in Wisconsin with supervision and hunters education. To allow a 4 year old to sit out there with an adult and actually shoot a rifle... sounds more like Dad was claiming one for the family and using the kid as his reason. I can not even picture a 4 year old that would or could do such a thing.
We read all the time about hunters caught with illegal deer, that were tagged by their wives, girlfriends or children of legal age, and only the hunter is around. It is just a way they can shoot more deer and hunt longer.