If you had the chance to work four days a week, but it meant you would not be able to work full time at your current job again, would you take it? I'm about to have 3 kids, might want 4, and would like to send them to a private school that would involve driving them/carpooling.
Also - would you keep your kids in daycare on your "day off" so that you could get your cleaning, food shopping, meal prepping, laundry etc. done? Or would you keep them at home with you? The cost of daycare is "affordable", but of course if would cut it in to our take-home income, so the benefit of me staying home without the kids to do these things would have to far outweigh the cost of sending them.
Re: WWYD: working 4 days a week
As far as leaving the kids in daycare all 5 days, I would do it. I work PT and both my kids are in daycare 5 days a week. It's great to maintain their normal schedule and gives me time to grocery shop and run errands that way our weekends are free. I can always choose to not take them a day (which I sometimes do during the summer).
I agree with others -- take the part-time and keep them in daycare 5 days. There's no law that says you have to send them every single time for the whole day, but it would give you a chance to efficiently get all errands and chores done, take one kid at a time to the doctor/dentist without carting them all, make your own appointments, etc etc.