I've been doing 32 hours over 4 days for the past month and I've found that I'm working more efficiently and with more attention to the work. I think it's the product of my subconscious treating the fourth day as a deadline every week. In effect, I think I'm getting as much done as if I was working 5 days a week, while at the same time getting paid for 4. If there is any merit to this, I can't see why every employer shouldn't opt for a 4 working day week.