Japan Life

It's completely optional for companies. If the government were serious, they would make it the law, not just a suggestion. They'd blast their entire population with a marketing campaign to make working 4-days a week cool or "kawaai". "work 4-days to get paid the same salary!", "be a good citizen and stay healthy", "help your family in need, say no to overtime!", "take care of yourself, say no to overtime!", etc.

They'd have a national hotline for workers to report their company for forcing overtime and create an investigative strike-force to show up unannounced at reported work places, then they'd take some kind of significant action e.g first a warning and mandatory training (paid for by the company), then another warning with mandatory training and a fine in the form of money disbursed to non-management staff, and finally a fine to the government, money disbursed to non-management staff, and a permanent presence of a government employee to monitor infractions.

Introduce something like that and I bet you in less than 5 years the attitude towards 4-day work week will change.