Another take on I-bonds or TIPS is that they can offset your annual increase in spending due to inflation. For example, suppose inflation is 10% and you are in your first year of FIRE with a $50,000 WR. The next year, you will have to spend $55,000. The third year $60,500.
Now suppose you started FIRE with $100k in I-bonds plus TIPS in your portfolio, an 8% AA for a 25x portfolio . Your I-bonds (assuming you bought with a 0% fixed rate like today) will pay 10% interest and reinvest that amount into the bond. Your TIPS will increase the principal value by 10% and continue paying their interest (usually a fraction of 1%) on the newly adjusted principal - for simplicity let's say they gain 10% in value each year too*. To make the TIPS comparable to your I-bond allocation, let's assume you reinvest all TIPS interest payments. Either way, and roughly regardless of how many I-bonds vs. TIPS you have, your $100k asset allocation to inflation-adjusted bonds increases alongside your increases in spending:
Year Spending Approx Value of Inflation-Adjusted Bond Portfolio
1 $50,000 $100,000
2 $55,000 $110,000
3 $60,500 $121,000
4 $66,550 $133,100
5 $73,205 $146,410
Cumulative spending over $50k/year due to inflation: $55,255
Increase in value of inflation-adjusted bond portfolio: $46,410
So you hedged 84% of the damage from five years of 10% inflation that hit at the worst possible time right at the beginning of early retirement with only an 8% asset allocation to inflation-adjusted bonds. You are still probably in deep shit because I don't imagine the rest of your portfolio did so hot in a world of 10% inflation that lasted for 5 years, but you at least hedged your drawdowns during that time, and in this scenario you're probably again at that early-1980s point where stock indices had single-digit PE's or you could just buy treasuries yielding 15% or so. So maybe NOW your safe withdraw rate is something like 8% or 9%.
Also, in a world of 10% inflation it is likely you'd be moving some assets into I-bonds during retirement at a pace of $10k/person per year. Just like now, that would be too good a deal to pass up. So maybe by year 5 you have allocated another 4% of your portfolio to I-bonds.
With the Federal Reserve showing signs of doing too little too late, maybe now is the time to implement such a plan to cover the contingency risk of high inflation. Then you use options strategies like protective puts and collars to mitigate the risk of your stock portfolio falling below a certain level. There's no simple way to model any of this stuff, but these strategies together seem to address most of the failure cases of the 4% rule: unexpected inflation and unexpected depression.
*in reality it seems TIPS have a fixed yield that is higher than I-bonds fixed yield. This is to compensate investors for the additional risk that TIPS have; their principal value can go down if deflation occurs, but I-bond interest cannot go negative.