I am missing the fourth (and fifth) level in the Navigation Pane in Word.
Is it possible to activate those?
By default, Word 2013 does not display styles
Heading 4 and greater until you use the highest numbered heading level currently displayed.
To add Heading 4 to the available styles, apply Heading 3 style somewhere in your document.
Heading 4 will immediately become visible in the Styles group on the ribbon.
Use Heading 4 and
Heading 5 will become available, and so on.