4 The Lord hath sworn, and will not repent, Thou art a priest for ever after the order of Melchizedek.
English Standard Version
4 The Lord has sworn and will not change his mind, "You are a priest forever after the order of Melchizedek."
4 God gave his word and he won't take it back: you're the permanent priest, the Melchizedek priest.
New King James Version
4 The Lord has sworn And will not relent, "You are a priest forever According to the order of Melchizedek."
New Living Translation
4 The Lord has taken an oath and will not break his vow: "You are a priest forever in the order of Melchizedek."
Matthew Henry's Commentary on Psalm 110:4
Glorious things are here spoken of Christ. Not only he should be superior to all the kings of the earth, but he then existed in glory as the eternal Son of God. Sitting is a resting posture: after services and sufferings, to give law, to give judgment. It is a remaining posture: he sits like a king for ever. All his enemies are now in a chain, but not yet made his footstool. And his kingdom, being set up, shall be kept up in the world, in despite of all the powers of darkness. Christ's people are a willing people. The power of the Spirit, going with the power of the world, to the people of Christs, is effectual to make them willing. They shall attend him in the beautiful attire of holiness; which becomes his house for ever. And he shall have many devoted to him. The dew of our youth, even in the morning of our days, ought to be consecrated to our Lord Jesus. Christ shall not only be a King, but a Priest. He is God's Minister to us, and our Advocate with the Father, and so is the Mediator between God and man. He is a Priest of the order of Melchizedek, which was before that of Aaron, and on many accounts superior to it, and a more lively representation of Christ's priesthood. Christ's sitting at the right hand of God, speaks as much terror to his enemies as happiness to his people. The effect of this victory shall be the utter ruin of his enemies. We have here the Redeemer saving his friends, and comforting them. He shall be humbled; he shall drink of the brook in the way. The wrath of God, running in the curse of the law, may be considered as the brook in the way of his undertaking. Christ drank of the waters of affliction in his way to the throne of glory. But he shall be exalted. What then are we? Has the gospel of Christ been to us the power of God unto salvation? Has his kingdom been set up in our hearts? Are we his willing subjects? Once we knew not our need of his salvation, and we were not willing that he should reign over us. Are we willing to give up every sin, to turn from a wicked, insnaring world, and rely only on his merits and mercy, to have him for our Prophet, Priest, and King? and do we desire to be holy? To those who are thus changed, the Saviour's sacrifice, intercession, and blessing belong.