A.T. Robertson Commentary Galatians 4:7

"So that thou art no longer a bondservant, but a son; and if a son, then an heir through God." — Galatians 4:7 (ASV)

No longer a bondservant (ουκετ δουλος). Slave. He changes to the singular to drive the point home to each one. The spiritual experience (3:2) has set each one free. Each is now a son and heir.