Fourfold Gospel Hypothesis: A Call for Discussion

I have a question for our authors and readers: Is the confidence in Markan priority, generally combined with acceptance of ‘Q’ as a source, justified?
I’m not asking whether you think Markan priority is likely, but rather whether you think the level of confidence in it amongst biblical scholars is justified.
David Alan Black has just posted a blog about it, which I copied to WhyFourGospels.com (allowing me to link to the specific post), discussing this very question. Dave’s position is not in doubt. I publish his book Why Four Gospels? in which he outlines his case for the fourfold gospel hypothesis.
I often avoid giving my opinion in these things, but let me just note that I believe one’s view depends to a large extent on how one weighs external vs. internal evidence, and, of course, your evaluation of particular forms of external evidence. If you favor internal evidence, then you’re likely to support Markan priority. If you think it unlikely that the church fathers either knew or accurately reported information about the authorship of New Testament books, you’ll likely support Markan priority. If not, well, not so much!
I did such study of the New Testament as I did under people who took Markan priority as a given. I was barely aware that there was an alternative, and paid it very little attention. One might think that my mind was changed by publishing a book on the topic, but that actually came second. My belief that Markan priority was essentially a given was shattered by reading William R. Farmer’s The Synoptic Problem. I followed that up with looking at some other material by him and others.
I am not convinced of everything Dave writes in Why Four Gospels?. I’m less concerned with issues of historicity than he is, I believe. (Note please that I did not say “unconcerned with issues of historicity.” I do believe that historical foundations are important.) But despite some remaining issues, such as that I do not see a convincing explanation for the state of the text of Mark, I think Matthean priority is more probable than that there was a hypothetical document ‘Q’.
So what think you all?
— Henry Neufeld