John Gill Commentary Galatians 4:1

"But I say that so long as the heir is a child, he differeth nothing from a bondservant though he is lord of all;" — Galatians 4:1 (ASV)

Now I say
To illustrate what he had said of the law's being a schoolmaster to the Jews until the coming of Christ, and then ceasing as such, he proposes the case of an heir during his minority, till he come to the proper time of enjoying his estate.

that the heir, as long as he is a child ;
anyone that is an heir to his father's estate, or another's, whilst under age, being reckoned as a child, as he is from his infancy to his manhood,

differeth nothing from a servant :
he is not his own man, nor at his own dispose; he cannot do as he pleases; he is under restraint; he is kept to school or to business, and is liable to correction and chastisement according as he behaves; nor can he have the free use of his father's estate,

though he be Lord of all ,
of all the servants, according to the Arabic version; or of the whole estate his father left him, of which he is Lord in right, but not in possession; he is right heir to it, though as yet it is not in his hands, nor can he do with it as he will.