Reading through this page, and one of the articles prev/next links weren't linking. Went to fix it, but finally decided it was correct (or at least consistent with other working posts), so I assume it is a bug in the markdown.
Part 3's prev and next links do work (for reference).
Part 4's prev and next links do not work.
Also note that the (presumably) JS preview for Part 3 shows it not working, but when generated, it does work.