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book about how we make decisions, not released yet:

Why is category 4 "the least important but most difficult to master category?" Why not category 3?
Mastering the category means you understand it, know what items belong in it, and can obey its rules.
I believe understanding category 4 is easy to understand, somewhat easy to know what items belong in it, but obeying its rules is difficult. I doubt it's specific to me. This is not to claim it's easy to follow the rules of category 3; it's not. Knowing you must let go (3) is easier to accomplish

Q: What was the biggest surprise you encountered while practicing this system?
A: How a savings account could help provide me happiness by allowing many items to be re-categorized. I kept realizing if I had more money available, then many possible current and future items could be less urgent. Would I care if I lost my job? Or should I say: Would I care if I lost my job but had two years of expenses saved? Money can also provide control in some situations. Money can't buy happiness, but it can be a great tool that provides leverage for achieving happiness if used properly.