Forbes has an article entitled "Why A Four-Day Work Week Doesn't Work" in which they come to the conclusion that so called 4-40s (Working 10 hours a day instead of 8 and taking 3 day weekends) don't work. 

But there are serious drawbacks. Packing 40 hours into four days isn't necessarily an efficient way to work. Many people find that eight hours are tough enough; requiring them to stay for an extra two could cause morale and productivity to decrease. As for saving on the cost of commuting, it likely isn't true.

37Signals writer Stephen Jenkins then weighs in saying Forbes has missed the point.  He says...

The point of the 4-day work week is about doing less work. It’s not about 4 10-hour days for the magical 40-hour work week. It’s about 4 normalish 8-hour days for the new and improved 32-hour work week. The numbers are just used to illustrate a point. Results, not hours, are what matter, but working longer hours doesn’t translate to better results. The law of diminishing returns kicks in quick when you’re overworked.

Besides, very few people work even 8 hours a day. You’re lucky if you get a few good hours in between all the meetings, interruptions, web surfing, office politics, and personal business that permeates typical work day.

Fewer official working hours help squeeze the fat out of the typical work week. Once everyone has less time to get their stuff done, they respect that time even more. People become stingy with their time and that’s a good thing. They don’t waste it on things that just don’t matter. When you have fewer hours you usually spend them more wisely.

Here's the problem with the 37Signals theory: It makes no sense.

Why exactly would an employee squeeze more work out of less time?  What's their motivation to do that?  I suppose this might work if you have a job that has absolute set amounts of work that have to be done in a week but most jobs have goals that can't be given a fixed time to completion.  Which means an employee who wastes time 5 days a week could just do less work and continue to waste time 4 days a week. 

Does Mr. Jenkins really see an employee who would normally waste time at the water cooler saying "Sorry, I can't talk because I only work 4 days a week instead of 5"?  Doesn't seem likely. 

I do think the idea has some merit if you're creative with it though.  At one point we discussed an idea where Friday would be a "meeting day" and if departments could get their meetings done in less time they'd get to leave early.  The idea was to take the #1 time waster and give them an incentive to do that quickly while at the same time removing it from the rest of the week (which in theory allows people to produce more during the first 4 days). 

Sadly that plan never got implemented.

I don't know, as someone who works about 66 hours a week it couldn't matter less to me in the end.  If given the choice I'd rather have a job that makes me want to be here 66 hours a week than a job that has me looking for ways to leave anyway.