A lot of smaller companies are open to it once you've worked there and proven yourself.
My current and prior jobs have people in 3-4 day a week roles. But the people in those roles are senior contributors whose value to the company is as much knowledge as it is contributions.
I would argue that while this is true, equally sometimes at interview you can be in a good negotiating position too, that's when they've decide they want you. Sometimes being wiling to work for a little bit under what they'd have been willing to pay, sweetens the deal. I mean, don't ask for 4 day week AND top dollar salary at the same time.. unless you're a proven superstar. ;)
My current and prior jobs have people in 3-4 day a week roles. But the people in those roles are senior contributors whose value to the company is as much knowledge as it is contributions.