We've been through that before, but ok, let's have at it again.
Before I start though, I must add as a disclaimer that I personally am inclined to believe that Paul indeed thought of Jesus as a man. I don't think Paul is a reliable source of that man's existence, since, you know, he's proud of only relating the stuff he's hallucinated about that man. But he may well have thought that his hallucination was about a real man.
Or maybe not. I don't know.
The problem with Paul is also that he's using highly metaphoric language all over the place. Or at least it's generally taken as metaphoric, because the alternative is crazy. E.g., since we're talking Galatians, in the same epistle to the Galatians he actually says that he, Paul, had been crucified, and is dead, and pretty much is now just animated by the spirit of Jesus. Needless to say, almost nobody insists that that one must be _literally_ what Paul believed, like they do for Gal 4:4.
So from the start I'm unconvinced by an argument boiling down to just stating certainty that exactly this is not a metaphor.
Furthermore the words he uses in places are actually more usually used in figurative senses when you look at the Greek usage instead of the English translation.
And in other cases, like Gal 4:4, it's more like a linguistic WTH. (If you're bored enough, you can check the original and word references here:
http://biblehub.com/text/galatians/4-4.htm )
The word Paul uses is "γενόμενον" (genomenon), a form of "ginomai", which isn't a straight up "born", but more accurately "happened" or "took place" or "appeared". It's a word for a state transition, rather than the usual word for birth, which was "gennaō" even for Paul in other places.
And it's interesting because Paul himself always uses "ginomai" in such senses everywhere else, but never for birth.
It's also used as meaning "to appear", e.g., a public appearance of someone. E.g., in Mark 1:4 or John 1:6.
Etc.
So the "under the law" part, we can pretty much ignore here, since Jesus appearing or revealing himself under the law makes just as much sense.
Which leaves us with the woman, and worse yet with an "ek" preposition, which pretty much means "out of". (Though this one too can be used in other ways and meanings than strictly "out of".) But still the slightly wrong verb. Curious.
The most usual argument against that one is basically that it may be an interpolation. Or at least that seems to be Doherty's favourite explanation.
Basically according even to Ehrman, that paragraph was a favourite place to interpolate a little more in various copies. The early Christians that favoured what would later become Catholicism had a problem with the Gnostics, who often made Jesus fully divine. Adding just a little more to make him more like the human Jesus they favoured, was apparently rather common practice.
And again, there is the issue that Paul never uses that verb for birth anywhere else. Paul has a rather consistent use of the two verbs, except in Gal 4:4 which stands out like a sore thumb. (And this is actually a very used criterion for spotting interpolations. E.g., it's the same reason we can be sure that the "doer" in the Testimonium Flavianum is bogus, because Josephus never uses that particular word to mean "doer". For Josephus it always means "poet", which would screw up the meaning.)
So it's possible that the "born of a woman" part was just interpolated in the first place, and the original only said that he appeared under the law.