My suspicion is that the time spent per week at work or 'doing' work is no longer the limiting factor for productivity in a vast swathe of knowledge economy jobs.
I have a theory that the human brain can only give-out so much creativity before it needs a bit of freedom to wander. I suspect that knowledge workers have big periods of time where their minds are effectively 'resting'/rejuvinating but these people are obliged to remain in the office and look like they are working.
Perhaps the four day week projects are showing that if people are given the choice, they can exchange some of that resting time for free time?