Police are under a duty to investigate.  Pending the results of that investigation either the charges will be dropped or the prosecutor's office will move forward on the charges.  The fact that the bruises were four days old may or may not factor in to this.  Possibly the girlfriend was somehow intimidated by your brother which accounts for her not reporting the incident earlier herself;  perhaps not.  The investigation will hopefully sort all of this out.What your brother needs to do now is consult with a criminal attorney.  Domestic violence charges are taken quite seriously, as they should be.