It is against the law for anyone to spank a child if it rises to the level of assaultive behavior. If you believe that your child is being assaulted then you should call CPS or the police. If you do nto believe that the spanking rises to the level of criminal behavior or child abuse you should file a motion with the Court to restrict the step-father's contact with the child. The Court might wonder why this has been going on for 4 years without you taking any action so you should be prepared to explain that. Get a good lawyer and discuss your options.