A.T. Robertson 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)

So long as (εφ' οσον χρονον). "For how long a time," incorporation of the antecedent (χρονον) into the relative clause.

The heir (ο κληρονομος). Old word (κληρος, lot, νεμομα, to possess). Illustration from the law of inheritance carrying on the last thought in 3:29.

A child (νηπιος). One that does not talk (νη, επος, word). That is a minor, an infant, immature intellectually and morally in contrast with τελειο, full grown (1 Corinthians 3:1; 1 Corinthians 14:20; Ephesians 4:13).

From a bondservant (δουλου). Slave. Ablative case of comparison after διαφερε for which verb see on Mt 6:26.

Though he is lord of all (Κυριος παντων ων). Concessive participle ων, "being legally owner of all" (one who has the power, ο εχων κυρος).