It would be really hard on working families that can't afford daycare.
What do these families do in the summers then?
Last I checked, public schools aren't childcare centers. They're educational institutions. Somewhere along the way, families got this notion that schools were there to watch their kids all day so they could go work. While some people have circumstances after having children which force them to work long and/or crazy hours (due to medical bills, loss of a previous job, divorce, etc), many parents have the same jobs they had before having kids. If they weren't prepared for the costs associated with having children, the fact kids will get sick and need to stay home occasionally (which in turn means THEY need to stay home and use sick/personal days, and not send their kid to school sick so they can save them for a vacation), the fact schools are there for education and not childcare, amongst others, they never should have had kids in the first place. If both parents are working before having the kids, they need to be prepared to make a sacrifice. Either they need to realize they are going to have to pony up for childcare if they want to keep their same careers, or one needs to put their career on hold until their child is older if they can't somehow work out the hours to accomodate when they aren't at school if childcare is prohibitive. Parents are supposed to raise their kids... not teachers, childcare workers, babysitters, friends...
As for 4 days or 5 days? If the 4 days also means making each day longer to make up for the lost 5th day, it is a horrible idea. For younger kids, the school day is already long enough. If schools wanted to go year-round, I think 4 days would be a very good way of breaking up the year for the kids. Otherwise, I'd think you'd have to have pretty much a week off every 5-6 weeks or so if they were going 5 days still.