Acting like unproductive hours are some random result goes against loads of companies who have been doing actual trials of 5 => 4 day weeks. Having time off is actually helpful, and the sort of procrastination from dissatisfaction/exhaustion/whatever that leads to this can be helped by this kind of stuff!

"Why is it 4 and not 3 or 2 or whatever" I mean it probably depends on people but TGIF is a saying for a reason