For one thing, it's looking at nearly risk-free rates of return while assuming a long-term horizon. (30 year treasuries are paying 3.65% now.) Sure it's easy to point out during the worst financial crisis of the last 70+ years that the S&P has fared poorly over the last decade. However even now, in a time of relative disaster, over the last 50 years (the timeline we're looking at if we're retiring at 30 and dieing at 80) it's returned about 8.5%. (That's using this chart http://moneycentral.msn.com/investor/charts/chartdl.aspx?sym... and a little math). Average since the Great Depression is more like 10%. While many people now believe it will be lower going forward even the worst skeptics are talkinng more like 7-8%. I don't know that I buy it because, as the old joke goes, economists have predicted 9 out of the last 5 recessions, but even if so that's a vastly different picture than 4%, and excludes the fact that there are many investments with higher ROIs to be found readily if you're either willing to do more work or accept more risk.
That's not to mention, FU money doesn't have to mean a jet-setting lifestyle. It just means enough to do what you want. One could live very happily in most of America spending $100k/yr, including their home. That's actually quite a bit when you consider that you're not paying income taxes, just capital gains. It's like making $150k, or 3x the national average. With $4m this is achievable even at money market rates.