Norway. I've only done this with a single employer over multiple years, but I've received offers from at least four others where I was clear about a 4-day week (with 20% reduced salary) being a requirement for me.

I've of course always taken care to not present this as the first thing that comes out of my mouth when interviewing, but I have always broached the subject in the first conversation I've had with decision-makers at the company in question.