I don't know where the 4% comes from but I've seen it in several places, including in an attractive offer from an educational establishment I once attended. App

I don't know where the 4% comes from but I've seen it in several places, including in an attractive offer from an educational establishment I once attended. Apparently, if I gave them 25x (pre-tax) the annual cost of maintaining a research student then they could create a perpetual studentship and call it the "Bloak Studentship" or whatever I preferred. So they think that it is possible to get 4% for ever in real terms from a well-invested untaxed fund. They've been doing this sort of thing for many centuries so perhaps they have an accurate long-term view.