For people that use the time to do errands, I suspect mid-week is better. I do wish we could settle on weekends though because the errands can be easily done on Friday or Tuesday or whatever, whereas a weekend trip cannot.
In the US most retail at least doesn't shut down on the weekend today. So a lot of companies having Friday off--which I see happening informally to some degree anyway (I almost never have a Friday meeting these days)--presumably wouldn't make a lot of difference. A day off in the middle of the week would mostly mean house chores for me without a real option to do a 3-day weekend which opens up a lot of possibilities.
while many have shared positive experiences, mine was quite the opposite. i did a 4-day work week for around 1.5 months and didn't find it to be beneficial at all. i went back to a 40-50 hour work week. i really love my work and want a lot more of it. not having a family also probably helps.
There has been a time in my life for each type of work arrangement, and there will be a time for retirement. Talking without that context is just going to result in talking past each other.