On most workdays I find myself so spent that a) I don't want to see people, b) I don't have any mental energy left. I can only do such things on the weekends, but then I'm also fighting with all the chores that have accumulated over the week.
A 4-day work week sounds very appealing because of the challenges mentioned above. An extra day with free and high-energy time would be amazing.
On 4 days I finally feel like I don’t have to spend my weekends preserving my energy. I can actually plan things to do without thinking “oh no, I shouldn’t, that’ll make me too tired for chores/work.” I can have a life!
edit: also, a benefit that people don’t tend to consider is that getting certain chores done is much faster on weekdays. You won’t find queues at the shops, bank, opticians, doctors etc on a Friday morning!
(And a bit also on how you count. Does spending time with your kids count as a chore or free time?)