Hebrews 4 discusses the promise of rest that God offers to His people. The writer encourages the readers to hold on to their faith in God and to not harden their hearts against His instruction. This rest is not just physical or external, but it is a spiritual rest that only God can provide. The chapter also discusses the power of God's Word and how it can reveal the hearts and intentions of people. Furthermore, the writer emphasizes the importance of having Jesus Christ as the High Priest, who can sympathize with our weaknesses and can intercede on our behalf before God.