I can't quite see schools going to a 4 day week, in college I could have pulled 10 hour days--but not in grade school. My attention span isn't long enough, and I'm not sure many kids in the lower grades could hack that. Although parents might like that, when you have both parents working.
For the summer my local school districts switch to a 4-day/10-hour week. It works out well since they don't have to worry about bus service and before/after school programs but in a larger picture, it might be too hard for large regions to switch during the school year because of how our bus service works and how much were used to planning our schedules around the 5-day work week.