Sounds like a good experiment. Let us know how it goes. I think there's a story about the Stoic philosopher Seneca who would reserve a few days each month to live with the minimal food / clothing just to expose himself to the condition that most people fear. Similarly, Ben Franklin, in his autobiography says that he lived for a period (I can't recall how long) on just bread and water while sleeping on the floor rather than in his bed, just to demonstrate to himself that he could do so if he needed to. Both of these examples are extreme of course, but if I understand your plan it is to live off of 4% of your current stash, meaning less than the 4% of your future FI number stash.
Perhaps a more direct correlation would be to use your planned FI stash baseline, but use the lower percentage that would represent your fallback spending level in a down market (3% or 2.5% rather than 4%). You might end up with the same number either way, but it seems clearer as an experiment on the planned scenario.