The four types describe individual pages. They are not automatically the top level of the navigation.
Organise the top level by what the user is working on, which usually means the product area, and use the four types within it. Top-level sections named Tutorials, How-To, Reference and Explanation force the user to know which type answers their question before they can start looking, and that is exactly the knowledge they do not have.
Every section needs a landing page that says what is in it and who it is for. A section that opens straight into a list of page titles makes the user do the sorting.
Cross-link between types wherever a user is likely to move between them: from a how-to guide to the reference entries it touches, from a tutorial to the explanation behind it, from an explanation to the guides that put it into practice. Those links are the main thing that makes a typed library feel like one document rather than four.
