I did the Old Testament/Psalms portion of my lectionary reading today from the Jewish Study Bible. The notes draw attention to the difficulty in separating Psalm 107 into the next book. The division between books 4 and 5 of the Psalms occurs between Psalm 106 and 107. But these divisions are later than the text itself.
One should be aware that the Psalms are a collection, and that they are individually composed. This makes their context within the book somewhat different in nature than the context of a particular chapter in another book. For example, when I look at a chapter in Samuel-Kings, I look for it’s place in the overall scheme of the history presented. In Isaiah or Jeremiah, while I realize that individual oracles were written at different times, I look for some sort of thematic arrangement. The Pslams are a bit looser than that, or at least we are less certain of just why the collection was arranged. Certainly, it is a collection of material by more than one author.
The Jewish Study Bible points out that Psalm 107 fits into the theme of Psalms 103-106, and indeed resembles them more than it does Psalm 108. They also suggest moving the word “Hallelujah” from the end of Psalm 106 to the beginning of Psalm 107. I would need to look at this further, but I am less impressed with that suggestion, even though I suggested that the Hallelujah at the end of Psalm 104 be moved to the beginning of Psalm 105 when I Psalm 104 article" href="http://rpp.energion.com/psalm104.shtml" target="_blank">wrote on it in graduate school.
That change would result in an envelope of Hallelujah around Psalm 105 and again around Psalm 106, while Psalm 103 and Psalm 104 have an envelope of “Bless the LORD, O my soul.” I think that single move I suggested back then works very well.
The thematic difference is more impressive, but I do see some thematic ties that point in both directions. I’m not certain this division should actually be changed, though we should realize it’s later than the original collection, if “original collection” is even valid in reference to the Psalms.
I’m going to link to Bob McDonald at Bob’s Log,who has done much more work on the Psalms than I have (and that’s an understatement!), in the hopes that he will comment.