Chris Vogler hands us clues to the fourth level as well as the third. (It's notable that on the first level, as well as other levels, you are given all the tools you need to solve all levels. It's only how you use the tool that determines its results for you.)
He goes into substantial work about the archetypes. Defining each in terms of their use in film examples. Very convenient for a modern world.
Vogler also brings up an interesting point - that the archetypes may be understood as simply masks worn by the characters. This explains various complex movie characters who seem to be different archetypes at different points.
Again, here we don't need to fall into semantic problems. One person's name for their understanding of God will not necessarily be used by another. Once you understand the purpose/function of an archetype, then it's appearance is easier to use as a symbol.
The other idea Vogler uses at the beginning is that the Journey our Hero takes helps find the various parts of personal beingness which are insufficient or lacking. The journey is for wholeness.
And this is why it becomes a route on the fourth level as well. But the main use (for this discussion) is on the third level - that of dreams and symbols.
So I say to study both Campbell and Vogler in order to fully understand these concepts and how they relate to solving life problems.
Obviously, if you are simply applying this to your own life in a pragmatic, first-level world, it becomes useful as a nice metaphor in the objective world - suitable for study and entertainment.
On the fourth level, the tool this book gives is to be whatever it is that apparently opposes or hinders your success. For only a lack of beingness on the holistic level is what keeps you in that problem or situation, or keeps that existing at all. Be all, solve all. (And again, I told you these levels all segue into the next, just as most people in the "real world" are intermittently psychic.)