I'm still not really seeing how the 4-day-week, at 5 days pay, is delivering the (almost magical) 20%+ productivity boost that its proponents seem to claim it does. I'm sure almost all people feel better when given the option to work fewer hours for the same money ... I know I would! How does that translate into increased productivity, though?
Having extra day to rest, do your hobbies, more time with loved ones ... etc __should__ contribute to less stress.
The extra day is not just about being at work, consider commute and other strings attached to going to work.