God the Father determined when the time was right for His Son to come to earth, as man and God. Revelation 13:8 says that the Lamb (Christ) was slain "from the foundation of the world." This world, the cosmos, is the world apart from God, and that world was founded when Adam and Eve sinned. When sin entered into God's creation, given God's purpose for mankind to be made into His image, it was necessary that there be a method of reconciliation between man and God. This reconciliation was only possible through the perfect sacrifice of Christ.
Galatians 3:22 says that the scripture has concluded all under sin. The totality of mankind is enslaved by sin and does not have the means to break free from its grasp. By "concluding" that everyone is under the bondage of sin, or under the curse of sin, the scripture shows that something external to mankind has to act in order that there be a solution to save man from himself and his sinful nature. This "conclusion" also demonstrates that none of the paths which man has embarked on—primarily, justification on the basis of one's own works—are of any lasting worth.
So when the "appointed time" (Galatians 4:2) had come, the Father decided to begin releasing mankind, in part, from the grasp of those controlling him, and the means of doing this was through the redemptive work of His Son. Roughly 4,000 years had passed since Adam and Eve's sin, and during this time there was ample evidence that mankind did not have it within himself to come up with a lasting solution which would bring about peace, harmony, and true unity with God or man. Sin was rampant, and mankind was destined to continue in sin and to reap the consequences. After 4,000 years of human history, nothing had changed in man's fundamental nature. God determined that this was a long enough period of time and sent forth the pre-existing Word as a man.
Paul emphasizes Christ's humanity when he points to the fact that He was "made of a woman." This attribute is universal for everyone else on earth, so we typically do not use it as a descriptor. But this descriptor illustrates that Jesus Christ was fully human. It also shows that Christ fulfilled various prophecies by being born rather than by coming to earth in all of His glory (Genesis 3:15; Isaiah 7:14; 9:6-9; Jeremiah 31:22; Micah 5:3).
Like all other men, Christ was "under law." This is not a reference to the Old Covenant; there is no definite article before "law" in the original Greek. He was not subject to the "Mosaic law," as some have assumed, but to the natural laws that God set in motion with the creation of man: He became hungry and thirsty when He went without food and water; He was wearied from physical exertion and lack of sleep; His physical body had limits in terms of the abuse it could take before it quit working; His body was subject to gravity, inertia, decay, and so forth. He was subject to every physical cause-and-effect situation that everyone else who has ever lived has been subject to.
Some modern translations render verse 4 as "born of a woman, born under [the] law." This is misleading, because Paul was not meaning to draw attention to the birth but of the supernatural conception. Paul uses the word ginomai for "made," and it means "to cause to be" or "to come into being." The emphasis is on the means or the action that something comes to be the way it is. The Greek word for "born" is gennao, which Paul did not use. Jesus Christ was "made of a woman" when He was miraculously conceived.
Christ was not "born under the law," in the sense that He was duty-bound to keep all of the ceremonies, washings, and sacrifices. However, He was "made under law." To be "under law" means to be subject to the condemnation of the law, which comes into action when one sins. Christ clearly never sinned, but nonetheless He was made [caused] to be "under law" when He was crucified and all of mankind's sins were laid upon Him, and He paid the death penalty which the law required.
Galatians 3:13 says, "Christ hath redeemed us from the curse of the law, being made [ginomai] a curse for us: for it is written, Cursed is every one that hangeth on a tree." This does not mean that the law is a curse, but that the law has a curse, and that curse is eternal death (Romans 6:23). Christ was caused to be "under law," under the condemnation of the law, when He accepted the death penalty for all of our sins.