You say $4000/mo for a couple = $48,000/yr – article says $15,730/yr for the worst off and $31,260/yr for the next worse off. Do you have a source for your figure?
0: https://www.fool.com/retirement/2018/08/21/heres-how-much-so...
"Roughly 40 percent of Americans who are considered middle class (based on their income levels) will fall into poverty or near poverty by the time they reach age 65, according to the study.
The study also concluded that if workers age 50 to 60 decide to retire at age 62, 8.5 million of them are projected to fall below twice the Federal Poverty Level, with retirement incomes below $23,340 for singles and $31,260 for couples. Further, 2.6 million of those 8.5 million downwardly mobile workers and their spouses will have incomes below the poverty level — $11,670 for an individual and $15,730 for a two-person household."
Which is substantially easier in your 20s and 30s than in your 50s and 60s.
Huh? In your 20s and 30s you’re paying to build your pension, paying for your mortgage, paying for your kids, paying for your student debts.
In your 50s and 60s you’re paying for none of that. What are you paying for at all?
It's pretty likely you aren't doing much retirement savings or paying for a mortgage on an income below double the poverty level at any age.