I have understood that 4% is a conservative guess of how much you can make yearly with money in a fund (preferably an index fund with cheap costs).

That way you should be able to take out 4% yearly, without your money declining.

My DH and I have been trying to calculate how much we will get in pension from the state and our pension funds, from the age of 67. It turned out that what we will get, is already enough to live off after 67, even if we would stop working today.

So now we just need to cover the period from 50 to 67 years: 17 years. We guess we need 400.000 NOK per year for a living, so we need 17X400.000 = almost 7.000.000 NOK, roughly 1 Mil USD, in spendable money. And a mortgage free place to live.