Rest in peace (4:9). Having opened his prayer with a sense of burden and oppression, the psalmist now can close it with a sense of confidence and peace. There is only one God, his Lord, in contrast to whom the “sons of man” are of no significance. But where the accusations of the sons of man had created that inner tension and anxiety which makes sleep impossible, the Lord granted that security within which sleep could be a time of rest and tranquillity. At the end, the psalmist has seen that he is better off than his adversaries. He had advised them to lie still on their beds, in an attempt to curtail their evil (v 5), but he could lie on his bed and sleep the sleep of peace which came from God.
Peter C. Craigie, Psalms 1–50, 2nd ed., vol. 19, Word Biblical Commentary (Nashville, TN: Nelson Reference & Electronic, 2004), 82.