Does their pay scale to reflect a 40 hour week even though they only work 4 and 3 days a week or do they have to work longer days to compensate for that? If pay doesn't stay the same they will have to get a second job.
But if you suggest they get paid the same for doing less then who compensates the company for paying someone them for a 40 hour work week but only getting 24 hours of work out of them? Because then not only are they paying that person more money they also have to pay someone else to pick up their slack which further increases the companies cost.
If this ever became a thing in America you'd see a shit load of age discrimination cases happen as companies let people go that hit the age of 50.