It's easy to see how it benefits you when only you're working 4 days/week but the rest of society on which your first world lifestyle depends, keeps working 5 days/week, being at your disposal for the same amount of time, while your enjoying more time off than the rest.
Need a doctor? Sorry, you'll have to wait more time now because now all are working 4 days/week. Need to go grocery shopping? Sorry, everything is closed today. Need a plumber? Sorry, most are off today as well.
There's no free lunch here. Moving the entire society to 4 days/week would require massive shifts economically and socially.
Meanwhile the number of doctors and plumbers etc. is not a fixed constant but driven by demand. There are plenty of qualified people currently doing useless work or unemployed who could fill those roles with the right incentives.
It is, but someone still has to work to keep society going in those "time off days". Doctors can't treat the same amount of patients in 4 days as in 5. Teachers can't squeeze the curriculum in 4 days. Customer support can't squeeze everyone in 4 days. Etc.
So how will you increase the number of doctors and other staff to work in the days the existing staff are off?
You can't just magic more workers out of thin air, especially now when we're already in a labor shortage in many fields like healthcare with everyone already working 5 days/week and with fertility declining in the west.
Examples of increases in quality of life include things like living into one’s 80s and 90s using dialysis and heart surgeries and new medicines.
Examples of changes in supply of resources include things like energy, labor, and a hospitable natural environment.
All of these things are constantly in flux, and automation sometimes outpaces them, and sometimes does not.