Talk to an attorney. If you filed an I-485 you should not be filing an I-539.
FORMER IMMIGRATION LAW PROFESSOR -- LEGAL DISCLAIMER: This answer is offered for informational purposes only. It does not constitute an attorney-client relationship.
It is not clear why you would need to change status to H-4, especially if you plan to work on an EAD based on your pending I-485. But if you ifle an I-539, the answers to the questions on the form are self-evident. An immigrant visa is something you apply for through the National Visa Center and a U.S. consulate or embassy. Are you applying for this? An immigrant petition is filed to seek an immigtant visa and permanent resident status. Has one ever been filed for you? Have you ever filed an I-485? You say you did, so you know the answer to that question too.