Hi Elizabeth, Mostly I agree with you in your last note - the first two Robert Abernathy's likely had more children than we know about but all we can go on is the known records and the likely suppositions we can make from some other records or what we think is most probably the case. What we know - from the 1757 Granville tax list - is that James Abernathy who married Elizabeth Cox was in the same household with two Robert Abernathys and in a lot of the cases this would make James the son of one Robert and the brother of the other. James also was the jurat on Robert Abernathy's 3 (Mary) will and was not on Robert Abernathy's 4 (Sarah) will because he drowned in 1785. I don't think James is the only one ever left off a will.
What I will say is that it is possible for James Abernathy who married Elizabeth Cox to be the orphan son of some unknown Abernathy in Virginia and was sent to live with his uncle Robert Abernathy in Granville County in 1757. In the absence of proof of this - right now I think he is likely Robert Abernathy 4's son. If all you had to look at was the 1757 tax list, you would most likely believe that he was Robert Abernathy's 4 son. I believe that is what is logical, however I do not have the kind of proof that would be considered 100%.
I might be the living, documented, male descendant of Robert Abernathy 1 of Virginia, I am A 21 on your Abernathy DNA web page. I will certainly be looking for more proof on James but I don't know if I will ever find it. I haven't looked in a while I need to look at your DNA web page.