Commentaries:
He is essentially asking, "Do you understand the instruction contained in the following episode drawn from Genesis?" Alternatively, "Do you comprehend what is in the law and its application to a New Covenant Christian?"
John W. Ritenbaugh
The Offerings of Leviticus (Part One): Introduction
The births of Ishmael and Isaac are recorded in Genesis 16 and 21. Though this happened long before the Ten Commandments and the other laws were given through Moses, Paul refers to this portion of Scripture as the law! Obviously, Paul uses "law" to mean the entire Pentateuch or Torah (the first five books of the Bible), not just the Commandments. In Galatians 4:24, he specifically mentions the Old and New Covenants.
Earl L. Henn
What Was the Law 'Added Because of Transgressions'?
Other Forerunner Commentary entries containing Galatians 4:21:
Genesis 3:15
2 Kings 4:8-37