Having a 4 day work week would mean one day off without kids (they are in school/sitter/day-care) which would be great and would give you tons more freedom just to get things done. Either chores around the house, need to take the car to the shop, visit the DMV, any number of things that are extremely difficult and busy on a Saturday with kids.

Of course, if everyone works 4 days a week and has the same 5th day off that advantage goes away I suppose.