My understanding is that once your H-1B has been approved, and you come back on a H-4, your H-1B is not valid. If I were you, I would obtain a H-1B stamp. Also I am not sure if you can go to Canada to obtaina a H-1B stamp if you are not a Canadian Landed Immigrant. Send an e-mail and see what the consulate will say. You will have to be very specific in your e-mail. Make sure you include, that you have only a H-4 stamp in your passport, and never had a H-1B stamp.