Personally, I am a big fan of the four day work week, I just do not believe it will maintain productivity levels. I would sooner believe a six hour five day week would.
Some people really, really like having a regular three-day weekend and will do just fine getting in their hours in four days. Others may well prefer a five day week with fewer hours per day.
If we could cut hours and give people choices as to how to allocate them, more people might get on board with the idea.