A four day work week is not beneficial to health. When work is allocated, it needs to be completed, whether you have five days or four to complete it in. A four day week introduces the possibility of four consecutive ten hour shifts, as opposed to five eight hour shifts. A longer than normal working day, therefore has health implications which can accumulate over a period of time.
Studies have suggested that longer working hours correlate with chronic diseases later in life. These were particularly substantial for women. Women working on average 12 hours a day were more likely to get heart disease, cancer, diabetes, and lung disease than their counterparts. A four day week may therefore push workers who already have a tendency to overwork, to crunch their work into a four day week. In addition to the physiological impact, there is also a psychological impact on wellbeing, and the ability to function productively is compromised.
Four day weeks may be counterproductive to the aims of providing more flexibility/leisure time. If a worker is to complete their tasks within four days, adding two additional hours to the working day will mean less time in the evening, for example, to relax or spend time with loved ones, especially young kids with bedtime schedules.