These two verses are parallel and synonymous. Most take the subject of I say (verse 4a) to be God; some, however, take it to be the psalmist (see Bible en français courant). Revised Standard Version represents the form of the Hebrew text. “To lift up the horn” means “to be arrogant,” “to show off one’s power” (New Jerusalem Bible “do not flaunt your strength”). Horn was a symbol of strength, power, pride (see 18.2 and comments). In many languages statements made after verbs of communication (I say to the boastful) will be more naturally expressed as direct quotations, as in Revised Standard Version.

The structure of the parallelism in verses 4 and 5 is somewhat different from anything encountered thus far in the psalms. Verse 4 follows the pattern of dramatizing and heightening the impact of line a by the use of a metaphor in line b. However, verse 5a then repeats the metaphor and adds on high, and then follows with a second figure, insolent neck. It may well be that the writer intended the structure to be chiastic (“boast … arrogant; arrogant … boast”). It is common for such structures to mark off a stanza, as is the case here. The poetic function of the form of these lines is also to give special emphasis to the warnings addressed to the wicked. Translators may contribute to the poetic meaning by employing local metaphors for boasting and arrogance rather than resorting to nonfigures as in Good News Translation.

An insolent neck (literally “a stretched-out neck”) is another figure for pride, arrogance, stubbornness. For the last line of verse 5 the Septuagint has “do not speak wickedly against God,” evidently translating the Hebrew word tsur “rock” (instead of the Masoretic text “with the neck”); this is preferred by Briggs, Weiser, and New English Bible.4-5 Hebrew Old Testament Text Project (“C” decision) stays with the Masoretic text, which it translates “do not lift your horns up high (that is, do not regard yourselves as important and full of authority), so as to speak with pride (literally, with neck) arrogantly.” But there is no need to depart from the Masoretic text.

Good News Translation has shortened and combined the two lines of each verse into one. It may be better in most languages to keep the two lines for each verse, as follows:
5 I tell boastful people to stop their boasting,
I command those wicked people to stop being proud.
6 I say to them, “Do not be proud;
quit talking so arrogantly!”

Biblia Dios Habla Hoy may also be used as a model:
5 To the insolent and wicked I say,
“Do not be so arrogant and proud;
6 do not boast so much of your power,
neither be so insolent as you talk.”

Quoted with permission from Bratcher, Robert G. and Reyburn, William D. A Handbook on the Book of Psalms. (UBS Helps for Translators). New York: UBS, 1991. For this and other handbooks for translators see here .