Albert Barnes Commentary Psalms 4:1

"Answer me when I call, O God of my righteousness; Thou hast set me at large [when I was] in distress: Have mercy upon me, and hear my prayer." — Psalms 4:1 (ASV)

Hear me when I call - When I pray. The word “hear” in such cases is always used in the sense of “listen to,” “hear favorably,” or “attend to;” therefore, in the literal sense, it is always true that God “hears” all that is said. The meaning is, “hear and answer me,” or grant me what I ask.

O God of my righteousness - That is, O my righteous God. This is a common mode of expression in Hebrew. Thus, in Psalms 2:6, “hill of my holiness,” meaning “my holy hill;” and in Psalms 3:4, “his hill of holiness,” meaning “his holy hill.” The psalmist here appeals to God as “his” God—the God in whom he trusted—and as a “righteous” God—a God who would do what was right, and on whom, therefore, he might rely as one who would protect his own people. The appeal to God as a righteous God implies a conviction in the psalmist’s mind of the justice of his cause, and he asks God merely to do “right” in the case.

It is not on the ground of his own claim as a righteous man, but it is that, in this particular case, he was wrongfully persecuted; and he asks God to intervene and to cause justice to be done. This is always a proper ground of appeal to God. A man may be aware that in a particular case he has justice on his side, though he has a general conviction that he himself is a sinner; and he may pray to God to cause his enemies to do right, or to lead those whose office it is to decide the case, to do what ought to be done to vindicate his name or to save him from wrong.

Thou hast enlarged me when I was in distress - That is, on some former occasion. When he was “pressed” or “confined,” and knew not how to escape, God had intervened and had given him room, so that he felt free. He now implores the same mercy again. He feels that the God who had done it in former troubles could do it again, and he asks him to repeat his mercy. The prayer indicates confidence in the power and the unchangeableness of God, and proves that it is right in our prayers to recall the former instances of divine intervention as an argument, or as a ground of hope, that God would again intervene.

Have mercy upon me - In my present troubles. That is, Pity me, and have compassion on me, as you have done in former times. Who that has felt the assurance that God has heard his prayer in former times, and has delivered him from trouble, will not go to God with the more confident assurance that God will hear him again?