Even so, just the reactions from Dutch people here on HN show that a lot of them are working 4 days a week. I do it too. My brother does it. Many of my neighbours do it. A 4 day work week is particularly popular with parents.
I'd love to work 4d/week, but how does it affect retirement fund buildup?
You build it up at 4/5 the rate that you otherwise would have.
More precisely: you build it up at a rate relative to your income, which is probably 4/5 of what it'd otherwise be. But if that 4/5 is enough for you, the retirement funds will probably be too.
Norway is the same.
The norm is 37.5 hour work weeks. We're entitled to a 0.5 hour lunch break every day, but that's unpaid unless you're required to spend it on-site.
It's even worse here in Germany where 37.5/40h work weeks are the norm but a lot of people, especially women, only find part-time jobs.
Which goes the other way, too: I'm somewhat of an exotic creature for daring to work part-time in Germany as a male and as a software developer.
Makes the stats look much more progressive than reality, even though 32/36 hour work weeks are common.