I've yet to see a single early retirement blog deal with the issues of investment risk, inflation risk and longevity risk properly. It's starting to drive me a tad potty. I will literally buy you a [starbucks] cookie and mail it to an address of your choosing if you promise to start using "Expected Annualised Real Return" instead of "average interest".
To be specific about the problem, a 4% annual interest over 4 years could represent: 2%,2%,4%,8%. Those will result in something different to a flat 4% pa. If you are going to have a withdrawal you need to work out what you do in the 2% years, because otherwise you have less money earning interest than expected and your gap starts to widen. Now what if the fed prints a crapload of money to lessen the national debt load, and you are suddenly facing $10 coffees?
Anything other than the risk free rate needs to be treated properly, as a risky return, or we are going to see in 40 years a lot of people who thought they'd saved enough for early retirement, but it turns out they had not.
Disclaimer: I am an actuarial student working in investment at a pensions consultancy. My living revolves around trying to quantify these risks.