If you remove category 4, you only have 15 questions that need to be answered. That trigger specifically looks each time the main board slide comes up, then decides if it needs to go to the results, or continue playing.
Depending on how you removed your slides, the results slide may still show a category 4. Click the top of the results slide where it says 'EDIT RESULT SLIDE'. This allows you to choose which slides are included in the results.
You don't have to change the passing %, but for a group like mine, if we use this as a quiz for a required content, we change the pass % to 70 or 80 depending on the subject. If you still want them to pass on 50, no problem. Your instructions would reflect 9000 max points, and needing 4500 to pass.
Hope this helps!