[onion2k]

As an employer there are plenty of jobs that I would be perfectly fine allowing the employee to work 20% less, as long as they cost me 20% less.

The point of the 4 day week is to decouple the link between output and time. Most employers aren't paying people for 5 * 8 hours of their time. They're paying them for "however much of X work you can do in 40 hours". But, and this is the key point, if tech has moved forward to enable people to do "40 hours of X work" in 32 hours then people could work a 4 day week for the same money that they're earning now.

For a large number of jobs that absolutely is the case, and people sit around wasting 8 hours a week doing very little because they can get all their work done in less time. Rather than them wasting the time, which doesn't benefit you at all, why not just work 4 days? People get more time for themselves, so they're happier, and you benefit from happier, more motivated staff. Everyone wins.