It's a two way street, companies have the right to have fixed schedules from their workers to ensure planning and everything else goes smoothly.
It is easier and sustainable to work around four-day work schedules than to give workers the flexibility of changing their hours.
Also of note, laws and regulations around the world differ in what full time means because full time is where the benefits kick in by laws; in other words; if the worker change to part-time, then the entire benefits package can be drastically impacted.
For an example, in many companies in US, if you work less than 40 hours, then companies are not obligated to give you any benefits such as paid time off, health insurance, etc, they can mark you as a contractor and not an employee.
Changing the 40 hours down to 30 hours as full time would not allow the companies to do that. So, I rather have four-day weekend with full benefits first and then part-time would be considered anything less than 30 hours.
Your idea means that it is more likely that the employees get less benefits while still requiring 40 hours work-load as baseline.