No silly questions!
If the middle book is intended to be the same length as the other three (give or take) I would take the Signpost that most speaks to you as an Author given the concerns of the characters and the dramatic material you want to cover.
For instance, if the Plot Progression is:
Understanding - Doing - Obtaining - Learning
I would set the Overall Story Concern of the first book to Understanding
the Overall Story Concern of the last book to Learning
and then the Overall Story Concern of the middle book to Obtaining
You'll note that this feels like a "better" plot progression for a Trilogy and I would venture to guess that those who can't break their books down into four parts are likely looking at a similar 3-Act structure for their series storyform and naturally want the middle section to be a bridge between two Signposts
In this example Obtaining would work better as an OS Concern if for nothing else than the primary focus, or Goal of the Overall Story, would fall into line with the end of the Journey - where they "end up" at the end of the story.
No rules, of course, but it's cool and I think sophisticated of the Author to align the format of their presentation with the actual narrative structure of what it is they are trying to say with the material.