Knowing Japan, it’ll formally be 4 10-hour days rather than 4 8-hour days. Though in a culture where it is considered unacceptable to leave before your boss does, whether that’ll translate into people spending an extra 2 hours in the office is uncertain.
It's max 40 hours a week before it goes into overtime, but lots of companies do expect unpaid overtime.
The nice thing is that once you're a salaried employee it's almost impossible to fire you without extensive work on the company's side. People only do the unpaid overtime if they want to advance in the company. If you're in a comfortable spot you can just not do it.
I mean no. Labor law in Japan in general is much like the West, only their culture basically forbids them from using it so they end up with the shitfest they have over there.
Hard working? Depends who you talk about, laborers like factory and construction workers sure and those kind of workers actually work pretty normal hours. But the stereo typical salaryman, who we all think of when we imagine an overworked Japanese worker, they don’t work hard. Japanese office workers are the most inefficient workers you will meet. Yes they work long hours but they need those to actually get shit done. Most of their day is wasted on socializing and performative work like wasteful meetings and basically just pretending to look busy.
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.