NET

4 because it is God’s servant for your well-being. But be afraid if you do wrong because government does not bear the sword for nothing. It is God’s servant to administer punishment on the person who does wrong.

Darby

4 for it is God's minister to thee for good. But if thou practisest evil, fear; for it bears not the sword in vain; for it is God's minister, an avenger for wrath to him that does evil.

KJV

4 For he is the minister of God to thee for good. But if thou do that which is evil, be afraid; for he beareth not the sword in vain: for he is the minister of God, a revenger to [execute] wrath upon him that doeth evil.