A Roman Catholic webpage states:
"Perpetual Virginity- Again throughout his life Luther held that Mary's perpetual virginity was an article of faith for all Christians - and interpreted Galatians 4:4 to mean that Christ was "born of a woman" alone."
This is another quote from the propaganda piece, The Protestant Reformers on Mary. In terms of polemical value, there's really nothing about this quote that shows "...the Marian teachings and preachings of the Reformers have been 'covered up' by their most zealous followers - with damaging theological and practical consequences." First, it's certainly common knowledge that Luther affirmed the perpetual virginity of Mary. Second, Luther's interpretation of Galatians 4:4 isn't in regard to perpetual virginity, but rather the virgin birth.
No Documentation is given.
4. But when the time had fully come, God sent forth His Son, born of woman, born under the Law,
5. to redeem those who were under the Law.
That is: “After the time of the Law had been completed, and Christ was now revealed and had delivered us from the Law, and the promise had been spread abroad to all nations, etc.”
Note carefully how Paul defines Christ here. Christ, he says, is the Son of God and of the woman. He was born under the Law on account of us sinners, to redeem us who were under the Law. In these words Paul has included both the Person and the work of Christ. The Person is made up of the divine and the human nature. He indicates this clearly when he says: “God sent forth His Son, born of woman.” Therefore Christ is true God and true man. Paul describes His work in these words: “Born under the Law, to redeem those who were under the Law.”
It seems that Paul insults the Virgin, the mother of the Son of God, by calling her simply “woman.” This bothered some of the ancient fathers, who would have preferred that he use the title “virgin” here rather than “woman.” But in this epistle Paul is dealing with the most important and sublime subject matter: the Gospel, faith, Christian righteousness, the definition of the Person of Christ, the meaning of His work, what He undertook and accomplished on our behalf, and what blessings He brought to us miserable sinners. The magnitude of this awesome subject matter was the reason why he did not consider the matter of virginity. It was enough for him to proclaim the inestimable and infinite mercy of God, that God saw fit to have His Son born of the female sex; therefore he mentions, not the worthiness of this sex but merely the sex itself. By mentioning the sex he indicates that Christ Himself was made a true man by birth from the female sex. It is as though he were to say: “He was born, not of a male and a female but merely of the female sex.” When he merely mentions the female sex, therefore, his phrase “born of woman” is the same as though he were saying “born of a virgin.”
Luther, M. (1999, c1963). Vol. 26: Luther's works, vol. 26 : Lectures on Galatians, 1535, Chapters 1-4 (J. J. Pelikan, H. C. Oswald and H. T. Lehmann, Ed.). Luther's Works (26:367). Saint Louis: Concordia Publishing House.