There are 52 weeks in a year (composed of 12 months), not 4 weeks a month .
4% is the amount of the monthly pay that is invested, not the compound interest rate (though 4% is still better than the ROI given by Social Security).
As for your "nobody making $7 and hour can afford to invest 4% of their money, well that is just plain ignorant. Uncle Sam is ALREADY taking 7.5%, my example uses only a little over HALF what the Fed is taking out of our hypothetical person's pocket right now. We are ignoring of course the 7.5 payed by your employer on top of your salary, that theoretically could be given to you instead.
This is a pure numbers exercise to illustrate what a lousy return Social Security gives in comparison to a real investment.