Technically 4 days would be better because of the commute and extended weekends for the employee. But as an employer 6 hours workdays would be the best, you get

Technically 4 days would be better because of the commute and extended weekends for the employee. But as an employer 6 hours workdays would be the best, you get basically the same value of your employees, since mostly office workers don't really work the 8 and also would make then free to study. You could also go crazy and get a second or third job.