There are likely several hundreds, if not thousands of companies already offering a 4-day work week or something similar.
For example, I work at a company called Forestreet (https://forestreet.com) and we already offer a 9-day fortnight.
The idea is that you condense your work week into 9 days, and take the 10th day off.
There are pros and cons to this model, but overall it's working very well.