I am Dutch too and it is common to work part-time for four days at a 20% cut [1]. However the four-day work week movement/experiments usually mean that people would work four days without a pay cut. The idea is (like going from the 6 day to 5 day work week at the beginning of the 20th century) that the extra leisure makes people more productive during work time. So, the same amount of work is done in less time, hence no pay cuts.
[1] In fact, if you work >= 6 months for a Dutch employer, you can request moving to a part-time schedule and they can only reject this request unless they have very good reasons:
You go from a 5 day workweek to 4 days, and take a 20% paycut. Sometimes it's possible to work 4 days of 9 hours, but it always comes with a paycut.