OK. But by that calculation everything else returns much less. Many potential investments (CDs, bank accounts, many US government bonds) have negative returns.

This is an all-in calculation, in which you get 4% on your money absolutely free and with zero risk (because all of the costs, risks, and even inflation are accounted for in the calculation). Put a dollar in today, and 18 years from now you'll have $2 to do with as you will, at today's prices.

Presenting the number as 4% only sounds low when you compare it to numbers calculated on a different basis.