Hello and Welcome to ‘Ask A Doctor’ service. I have reviewed your query and here is my advice. The Robitussin contains an over-the-counter cough suppressant(dextromethorphan) and an antihistamine(chlorpheniramine). Virtussin AC contains a prescription cough suppressant(codeine) and an expectorant(guafinesin). The antihistamine is useful for allergic illnesses, such as hay fever. The expectorant is supposed to help the patient expel mucus from lungs and throat. Neither one of these ingredients is for cough. For many years I have been reading about cough suppressants(both over-the-counter and narcotic). The information has stated that these medicines are not very effective for cough. If you feel that you must give her one of these meds, then I suggest the Virtussin AC because it may have a better chance of helping her. Several years ago I came across a medical study that showed giving 1-2 teaspoons of honey at bedtime did help the patients in the study cough less during the night. Thank you for including the names and contents of these drugs in your comments. Don t forget to keep her well hydrated. To answer your question, it should alright to give the Virtussin AC after waiting a full four hours from giving the Robitussin. Hope I have answered your query. Let me know if I can assist you further.