BOOK Four (Psalms 90 to 106) forms a unit with Book Five (Psalms 107 to 150) just as Books Two and Three were closely connected. A striking literary evidence of this grouping is the choice of name for God used in these books. Books Two and Three prefer Elohim as the name of God and tend to avoid the name JHWH. Books Four and Five prefer the name JHWH and avoid the name Elohim. In fact, the word Elohim is used only seven times in Books Four and Five. Of these seven uses of Elohim, six are in Psalm 108 and one in Psalm 144. Both these psalms are mosaics of material from previous psalms.
Another characteristic common to Books Four and Five is that the most of the psalms here do not have a heading ascribing them to any author.