This Elchasai puts forward as a decoy a polity (authorized in the) Law, alleging that believers ought to be circumcised and live according to the Law, (while at the same time) he forcibly rends certain fragments from the aforesaid heresies. And he asserts that Christ was born a man in the same way as common to all, and that Christ was not for the first time on earth when born of a virgin, but that both previously and that frequently again He had been born and would be born. Christ would thus appear and exist among us from time to time, undergoing alterations of birth, and having his soul transferred from body to body.
http://www.newadvent.org/fathers/050109.htm
So I think that Paul, by "born by woman, born under the law" (Gal 4:4), was insisting, against this diffuse tradition of multiple earthly appearances of Jesus, by claiming that Jesus was born only one time, precisely "in the fulness of time".
The "fulness of time" was a negative fulness, for Paul. It was the time of maximum power of the archons-demons. That time was when the Law was given ("born under the Law"). Not by god but by the archontic angels. (Precisely when Moses did and raised/impaled the serpent, de facto cursing him?).