What I am saying is that as long as an heir is underage, he is no different from a slave, although he owns the whole estate.
The heir is subject to guardians and trustees until the time set by his father.
So also, when we were underage, we were in slavery under the elemental spiritual forces of the world.
References for Galatians 4:3
But when the set time had fully come, God sent his Son, born of a woman, born under the law,
References for Galatians 4:4