I believe it is not safe at all. A 4% withdrawal rate would require the US stock market in the 21st century to produce returns similar to those of the 20th century, i.e. in the vicinity of 7% in real (inflation-adjusted) terms. Fair estimates for stock returns going forward are not this high. Rick Ferri proposed a real 5% over a 30-year horizon in 2015. I recall William Bernstein in his book "Rational Expectations" (2014) proposing a real 3.6% over the very long term. These long-term estimates are based on the Gordon equations. According to this model, the long-term growth of a 100% SP500 stock investment in real terms would be the current dividend yield (~2%) plus the expected *per share* dividend growth rate (often quoted at 2%).

It is interesting to look at what happened to other countries in the past. This graph shows that very few countries could sustain a 4% withdrawal rate, and at 100% stocks no country did. Take a country that was not devastated by war in the 20th century like Switzerland. With a 100% stock allocation, 3% would have been too much. In this century, it seems likely that the US will be like Switzerland was in the 20th century, rather than like the US in the 20th century given expected stock returns. Notice that except for Spain the countries that did not support a 2% withdrawal rate were all devasted by WW2. Spain had a civil war in 1936.


[![enter image description here][1]][1] Image source: https://www.bogleheads.org/wiki/Trinity_study_update

William Bernstein's opinion on this subject is as follows: "Two percent is bullet-proof, 3% is probably safe, 4% is pushing it and, at 5%, you're eating Alpo in your old age [...] If you take out 5% and you live into your 90s, there's a 50% chance you will run out of money." ([Source][2])


  [1]: https://i.sstatic.net/4Axpp.jpg
  [2]: https://www.bogleheads.org/wiki/Safe_withdrawal_rates