Some work I did today on the "book" divisions in the Psalms.
Here Radak says he's wrapping up Book III, praise to the shaper and doer (i.e. the creator), and now this is Book IV.
Scroll through for the other book divisions.
The one just before 90 has a different verb גמר (not שלם) and another doxology, and introduces Book IV quite simply with זה, which seems rather flippant to me!
These are all from www.nli.org.il/en/books/NNL...