"while someone who is 80 could withdraw 4% (80 ÷ 20 = 4)"
According to recent actuarial tables a males life expectancy (ie 50 percent chance of making it) at 80 is only 8.2 years and a females 9.6.
I think you can be a bit more aggressive.
The reality of this made me LOL.
I, too, think 4% is probably just fine. And... if it isn't, it isn't just you hanging there out on a limb. If it isn't, then something really terrible happened and we are all just royally screwed in the tookus.
It's one of those bets like "if the world goes to hell, I need to have a bunker full of ammo, an army and 16 fully automatic weapons" vs "oh crap, it's gone this far? Screw it."
4% is reasonable, IMO. I am now looking at 2% and thinking... I could probably have had 5-10 more years of freedom. I could still have 20+ years of absolute over-the-top hedonism..... Ah well....