> Big investments generate quite a lot of money — you can draw off about 4% of an investment every year without depleting the principal, because you get back that much or more in interest.
That is not at all what the 4% rule they linked is talking about. The 4% rule means, roughly, from the original source, "95% of the time you won't completely run out of money in 30 years". Indefinite withdrawal from an endowment is a very different problem.