At least part of the 4% rule at least historically stems from the fact that returns at or above 4% have historically been achievable with less volatile investments like bonds, maybe even savings accounts, or annuities for people who are willing to give up some control for a guarantee.
That fact that all those investments have been rough on returns recently or at least the past couple years with bonds coming to a crawl too as interest rates trickle up puts some retires in a somewhat long term pickle since having most of their money in say a bond index is not going to sustain a 4% withdrawal through earnings.
The rules for someone who is traditionally retiring with at best 30 years of life left in them and an ER are a bit different to begin with. A 20 or 30 something ER isn't going to use an annuity.
Also someone this young in tough times could take up side jobs and let their investments sit untouched for a year. MMM, when he talks about 4%, it seems he is looking at a more sustaining withdrawal in a more equity based account, where as most traditional retirement calculations expect you to eventually deplete your investments and maintain a steady withdrawal rate.
The philosophy of ER doesn't seem to preach never work a day in your life again and never run out of money. It is more like, you don't have to work. So save enough money that you can live off investments most of the time, but be flexible enough to pick up some side work to keep your nest egg stable or growing.
