I swapped to a 4 day week (with relevant pay cut, which certainly made it easier to justify with my employer) in order to finish my MSc without burnout and it's changed my life. The entire balance of the working week has changed - 4 days is almost a 50-50 split which makes an enormous difference to your psychology (0.57:0.71). My employer has stated they've not noticed any change in productivity either, including on…
So your employer is getting the same work out of you with less pay? Sounds like you should push for your old pay back.
Outside of knowledge work, the 4-day week appears where demand outstrips supply. It's pretty common for dental offices, an one example, to work 4 days _just because they can_. The bos can pay themselves several hundred thousand dollars a year and pay their staff competitive salaries without giving up their long weekend plans.