When Jesus gave up His glory to enter into the physical creation, He took on the form of a slave. (redeeminggod.com)
Though he was in the form of God, he did not regard equality with God as something to be exploited, but emptied himself, taking the form of a slave, being born in human likeness (Phil. (religion-online.org)
While the lectionary matches Hebrews 4 to this reading, I find a closer connection to Philippians 2, where Paul speaks of Jesus as one who was equal with God but «emptied himself, taking the form of a slave.» (religion-online.org)