In practice, every company is open to a 4-day work week, if you're valuable to them and the alternative is losing you. Some companies are just more likely to extend the same benefit to everybody.

Definitely do it, by the way. I dropped down to 4 x 8h when my second son was born, and simply never switched back. My quality of life is a lot better now, I don't seem to ever get sick anymore, and I get 3-day weekends every single week.

Way better than having an extra few grand on a paycheck.