During my career, I have worked for a few small as well as big companies where the management indeed encouraged limiting the working hours to about 4 to 6 per day. All of them had 40 hour week on the contract or legal agreement but the company culture went beyond the written contract to support something like a 25 hour week.
In my career, I did not see any kind of correlation between financial success of the company, productivity and the number of working hours. Financial success had a strong correlation with good decisions made by higher management. Productivity had a strong correlation with high skill-level of engineers and managers hired.
Another way to describe what I mean is: When I worked for one of these companies that encouraged four-hour working day, there were competitors who worked the regular eight-hour working days and were beating us in the competition, and there were also competitors who worked the regular eight-hour working days and were losing to us in the competition.