I took over-the-counter Robitussin (guaifenesin) can I take prescription Robitussin (guaifenesin) with codeine after 4 hours?

Probably . Remember Robitussin (guaifenesin) mainly works by irritating the stomach to stimulate secretions. Taking more often than 6 hours my lead to stomach upset, so you might want to wait a little longer than 4 hours.