You seem to be saying that the market return smoothed out will be 4%? Its not clear but it doesn't seem to be an "interest rate" to me.
What I'm saying is that, historically, real assets (equities, real estate, etc.) have outperformed bonds and cash by an average of 3% to 6% a year. When you think of it, such assets must give a higher return. Otherwise, people would just stick their money on deposit and not take a risk. The extra return from real assets is higher than is justified on purely rational economic grounds. This is because of the psychological phenomenon of loss aversion: people experience greater pain from losing money than the joy they experience from gaining the same amount. Thus, they have to be compensated when there's a risk of losing money. That was true in the past. It will equally be true in future.
Assuming average out-performance of 3.5% per annum (which is at the lower end of the above range) from such assets, and assuming bond yields of 2% per annum, we arrive at an average return (before fees) of 5.5% per annum. Deducting 0.5% annual fees, the net return is 5% per annum.
The return in any year could be higher or lower than the average but I've minimised - almost to the point of completely eliminating - the risk of any individual investor losing money, even in the short-term, by spreading the return over many years. Back testing shows that, on reasonable assumptions for cash flows, there would have been a (small) negative return in 1974, but not since then.
In normal circumstances a smoothing approach on the lines proposed wouldn't work, because smart operators would buy when smoothed values were below market values and sell when they were above them, but I've suggested a number of simple rules, both for money coming in and for money going out, to prevent that happening.