While I agree that someone working a 4-day week is more than 80% as productive as someone working 25% more hours, I don’t agree that a “4 day workweek usually means…without loss of compensation”. In many cases, you can’t possibly know what would have happened (if an entire company is 4-day schedule) or a company transitions to a 4-day workweek instead of paying raises or while paying smaller raises.
I can’t think of any place where the expected schedule for one employee is 32 hours and another, otherwise identical, employee is 40 hours and those two are paid the same.
To me, a 4-day workweek just means you have scheduled/expected work on 4 days and none scheduled/expected on the other 3.