Is it safe to take nyquill 4hours after robitussin (guaifenesin)?

If used correctly: If one is using nyquil and "plain" Robitussin (guaifenesin) correctly, following the packages' instructions, and has not had any problems from either medication alone, then one would expect no problems from taking them together (at the same time or at different times). The nyquil does not have the same ingredients as plain Robitussin (guaifenesin) has, so one would not be doubling up on an ingredient.