Except for a really strange time-line (that left out Spain and North Africa completely), I suppose this is possible although the claim that none of these women have a non-Jewish descendant is not likely at all ~ probably half the people who post/read FreeRepublic know several people descended from these four women and who would certainly be surprised to hear of any Jewish ancestors.
Yeah, mathematically it doesn't add up. All we need is for one of those 40% to intermarry and have a child, and boom, the claim's exploded. And logically speaking many of them already have intermarried and had children, certainly within the last 80 years, and sporadically well before that.
The Jews who lived in Spain are not Ashkenazim--their descendants are the Sephardic Jews (who continued to speak Spanish into the 20th century in many cases).