How can the Law be called fleshly in Galatians 4 if it is God's spiritual Law?
God's Law is spiritual and perfect in itself, but in its setting, usage, and practice it was fleshly because it depended entirely on the activities of the people who were under it. Paul's allegory uses Hagar and Ishmael to picture that fleshly, bondage-producing administration at Mount Sinai.
Les explains that Ishmael was of the flesh by way of the slave girl, and the allegory is that the Law given to Israel through Moses was fleshly and carnal. Yet he cautions not to misunderstand: the Law was spiritual and was God's perfect Law. The point is that in its setting, usage, and practice, it was fleshly because it was all dependent upon the activities of the people under that Law. The allegory then identifies Hagar with Mount Sinai in Arabia, answering to Jerusalem that now is and is in bondage with her children. At the time Paul wrote Galatians, about 58 A.D., the Temple was still operating, Israel was still under the Law, still practicing it, and sacrifices were being offered by the thousands. By contrast, Jerusalem which is above is heavenly, free, and the mother of us all, referring to Sarah, the free woman who had the child by promise. She stands in contrast to the Law as it was given to Moses at Sinai.