A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
psa 4:1On Neginoth, that is, stringed instruments, as the kind of musical accompaniment. On other parts of title, see Introduction., The historical occasion was probably the same as that of the foregoing [see on Psa 3:1]. The writer, praying for further relief, admonishes his enemies of the vanity of attacking God's servant, exhorts them to repentance, and avows his confidence and peace in God's favor. (Psa 4:1-8)
Hear--as in Psa 3:4.
God of my righteousness--or, "my righteous God, as my holy hill" (Psa 2:6), who will act towards me on righteous principles.
thou hast enlarged--expresses relief afforded in opposition to "distress," which is expressed by a word denoting straits or pressure. Past favor is a ground of hope for the future.
psa 4:2sons of men--men of note or prominence (compare Ch2 21:9).
turn my glory--or, "royal dignity."
into shame--or, "reproach."
vanity--a foolish and hopeless enterprise (Psa 2:1).
psa 4:3godly--an object as well as subject of divine favor (compare Psa 105:14-15).
psa 4:4Stand in awe-- (Eph 4:26), from Septuagint, "be angry." Both clauses are qualified by "not."
psa 4:5Not only repent, but manifest penitence by sacrifices or righteousness or righteous sacrifices, &c.
psa 4:6Contrast true with vain confidence.
light of thy countenance upon us--figure for favor (Num 6:26; Psa 44:3; Psa 81:16).
psa 4:7corn and wine--literally, "new corn and wine."
increased--an abundant harvest giving great joy (Isa 9:3).
psa 4:8both lay me down, &c.--or, will lie down at once, and sleep in sure confidence and quiet repose (Psa 3:5).