We love our 4 day weeks! We have weekly morning medical appointments for my daughter and tried doing school in the afternoon that day each week. Didn’t work out so well, as they’re much better focused in the morning esp. on the skill based subjects. After too many grump days, we just use that day as a day off. We’ve done Mondays off for some terms and Fridays off other terms. It’s never seemed to affect their learning and attention the rest of the week. As a child, I remember always wishing I could have Monday or Friday off with the weekend. Seemed like Saturday was spent doing errands and garden work with mom and dad and Sunday was for Church, so there never was much of true day to play. My kids are also so much more refreshed for their week with that extra day off each week.
Some weeks, it feels like one less day makes everything more squeezed. To help with that, we school closer to year round as well as often read a history lesson or composer story or literature read-aloud, etc at dinner. Low-key Saturdays are great times for art lessons and nature study. If you go with a 4 day week, it doesn’t have to all fit in from 8 to 12 those four days, 36 weeks a year either 
