Joking aside, the amount of hours put in by self employed people for very little pay (relatively) is a hidden travesty.
It makes me cringe when people say "...it must be great owning your own business and just doing your own thing, bet you make a fortune!"
I’ve been both self-employed and salaried over the years - nobody’s better off, really. Salaried workers also do large amounts of unpaid work, especially in clerical and technical jobs, but have a lot less flexibility in when that unpaid work is done. Self-employment also allows you to recoup your work expenses far more easily: I have had employers who refused even basic subsistence payments for out-of-hours work, or who insisted I undertake long-distance travel on weekends without compensating me for the loss of my free-time - they didn’t stay employers much longer, though. Plus, the file-and-pay tax regime can really benefit you financially if you’re able to budget when compared to PAYE.
On the other hand, the surety of income for the next 12 months (most private-sector “permanent” jobs are just rolling annual contracts) you get with salaried employment is something most self-employed workers can only dream of, especially when you try to get a mortgage, and if you do find yourself unemployed, you can at least get what meagre support the state offers; self-employed people on the other hand are never considered unemployed without proof of cessation of trading.
On the original point, for “office work”, moving from 5 x 8 hours to 4 x 8 hours probably would increase productivity overall, but it’s not a magic bullet: to get the benefits would also require considerable workplace culture changes, such as a rigid enforcement of those work hours and an end to out-of-hours business communications, to prevent the new four-day week becoming 4 x 10+ hours a week.
A friend of mine with decades of project management experience in finance, telecoms, pharma and IT businesses reckons on about 25~30 hours of productivity per employee per week when assessing resources, regardless of the number of hours actually spent “at work”. I would concur: I have done jobs where I put in 60+ hour weeks consistently, and I might have even been productive for 50 of those hours at first, but after a while, fatigue sets in, and you’re back down to lower net productivity due to a higher rate of errors. Then, after whatever deadline is met, there’s invariably a crash period, where total productivity tanks, sick-leave ticks up, and the employer ends up paying off those extra hours at a very high interest rate. (To add insult to injury, whenever an employee gets wise to the stupidity of the company’s death-march practices and quits, they call it “burn out”)