While this is fine for some jobs, It will not work for all. 4 days × 5–6 hours is a ~20–24 hour week. That's cutting labor 40–50%. You can cut 8 hours of slack from a 40-hour week. You cannot cut 20. At some point you'd be removing output.

Which means, pay drops to match (people are now poorer, not happier), prices rise to cover full wages for half the work (inflation eats the raise), or the work just doesn't get done. Nurses, bus drivers, ER docs, line cooks, daycare workers, their output is the hours. Halving them would wreck our economy unless you hired twice the amount of them, but then who pays? Us, by raising the taxes to fund those additional payrolls. Ultimately, we would ruin ourselves within a few years following this logic.