I think we're talking about after-tax money; unless you live somewhere that has a wealth tax, once you've paid all your taxes on income (wages or capital) then you're free and clear.
Common retirement strategies, like a 401K, differ taxes until withdrawal. So taking out 4% of your portfolio every year upon retirement would in fact incur taxes. Since a 401K is massively tax advantaged in your highest earning years it only makes sense to maximize this portfolio while you can thus delaying, but not avoiding taxes, until a later date when you'll likely pay much less on the income.
