where can one find such 4-day work week jobs? Jobs listed on most portals are either full-time or part-time (contract basis)
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.
It should probably be a norm but right now it's ad-hoc for companies to support it in order to retain staff.