The last verse is one of the most well known in all the Psalms.
In peace I will both lie down and sleep; for you alone, O LORD, make me dwell in safety. (Psalm 4:8)
Why is it that he can lie down and sleep in peace?
What it’s not
It is not because all of his prayers are answered right now (4:1).
It isn’t because he is comfortable and treated like he wants to be treated (4:2).
He knows some things that allow him to sleep in peace.
What he knows
The Lord has answered his prayers in the past (4:1).
The Lord cares about his people (4:3).
The Lord has given him so much joy in the past (4:7).
The Lord is his safety, and apparently the Lord doesn’t sleep (4:8).
Be silent and wait (4:4). Your silence doesn’t mean his action will be weak (Exodus 14:14).
Ponder (4:4), but what? You can start with the things David knows and says here. If the Lord answered his prayers, he can and will again. If he cares about his people, he cares about your current situation. If he gave joy in the past there is hope for the future. If he is our safety then he doesn’t sleep when we sleep.
Do the next right thing (4:5) because many are looking for something good (4:6). Your good deeds point beyond themselves (Matt 5:16).
Trust the Lord (4:5).