Here's why:
* DVD releases were not organized by season volume. The DVD sets were typically released as full seasons or grouped together in multi-season collections.
* Season 4 was a tumultuous time for the show. The series went through significant changes, including the separation of Jon and Kate, which may have impacted how the content was packaged for DVD.
* The show's popularity declined. The show's later seasons, including Season 4, were less successful, leading to fewer DVD releases.
However, you might be able to find individual episodes of Season 4 online or through other streaming services.
If you're looking for a specific episode or storyline, it's best to search online for "Jon & Kate Plus 8 Season 4 episodes" to find streaming options.