This is a fantastic idea. You are correct in saying this works during career change negotiations. It does not work if you work for a very small company.

I worked in a small printing firm where a co-worker and I negotiated leaving early, taking a pay cut during the slow summer. Pressmen were allowed to leave if they finished their print schedule. My co-worker and I were creatives and wanted to work on our own projects.

This idea bred jealousy and contempt from the other owner. Even though a year prior a new employee negotiated leaving early on a couple of days. He too was a creative artist. He stuck to the agreement he made with the owners. Somehow this did not work for us. This lasted two weeks. The other owner saw this a way for her to leave early and leave us her work. And then the two owners went on their three week vacation.

If you are commuted to working with a company and wanting a four day work week it is at the time you are negotiating in the hiring process that you can leverage this idea.