It's a 4% WR the first year. After that, the actual WR will vary even while the person withdraws whatever the previous year + CPI
Because the stash will change, imagine if stocks and bonds fell 50% the second year. The person would still withdraw $40k plus CPI. But the withdrawal Rate would be around 8% due to the smaller denominator.
Sure, but the number doesn't tell you much, especially if you take no action. What I care about is the (S)WR. That methodology already accounts for investment variance.
What I assume is that if I make it to Social Security then I will follow something like Audreyh1 "I use the % of remaining portfolio method which is based on prior Dec31 value and takes the same % every year." At some point I will be less interested in my potential terminal value and more interested in maximizing my spending/giving.