Minikitten has an interesting idea, though I think I would have to do it opposite. I would rather start off strong and have the days become shorter as the week progresses.
I think four tens would be fine with me. It seems most people who have done it think it is great.
I hear we work eight hour days because the day is broken into three equal parts: one for work, one for sleep, and one for free time. The logic is flawed, however, because most of us actually have nine hours between the time we start and time we finish working (one hour for lunch in the middle, which I don't consider an hour of free time). Not to mention time to commute and get ready for work. This leaves us with very little free time during a workday.
One less lunch in the middle of a workday, one less day of commuting and getting ready for work each week would add up.