Which one of the following is correct in respect of marrying a fifth wife while the four wives are…

A Muslim man may have at most four wives at a time. Under Shia law a fifth marriage while four wives are living is void (batil), while under Sunni law it is only irregular (fasid) and becomes valid if one of the earlier marriages ends. So it is neither void in both schools, irregular in Shia law, nor valid in Sunni law.