I think that while it would be wonderful to have the extra day off, there are a lot of problems.
The biggest thing that I see is that my children are already exhausted from being at school from 7:45-3:30, if we extend that day until 5:00, would I be forced to push their bedtimes up to give them the rest that they need to function all day?
What about homework? Right now my first grader has about 30 minutes of homework and reading each night. If we got out at 5:00, home by 5:30, dinner by 6:00, homework at 6:30, bath at 7:00, that gives him exactly 1 hour, if that all runs perfectly to relax. In my opinion that is too much for a 7 year old kid! Kids need time every day to play and just be kids. They can't "bank" that time on Fridays and use it up during the week. Kids need to be in school yes, but they also need to be ready to learn while they are there.
What about extracurricular activities like football, band etc...who compete/perform? Would you really want to have a kid practicing another hour a day in addition to their already over-filled day? What about games? If your district is competing against other districts, you would have to play on an agreed schedule. Around here, that means Friday nights.
In my opinion, and it is only an opinion, the decision to go to a 4 day week and add time to each day is not going to improve education and the potential cost savings are not worth the actual costs.
As a teacher, I would LOVE to have an extra day off per week but it just isn't what it best for kids, at least if they are going to add to each day.