If 4-day workers are 25% more productive than 5-day workers, or 5-day workers are 20% less productive than 4-day workers, then no. While it's hard to increase

If 4-day workers are 25% more productive than 5-day workers, or 5-day workers are 20% less productive than 4-day workers, then no. While it's hard to increase productivity by 25%, it's not a problem for workers to drop productivity by 20%. Owners can work as much as they want, of course.