Even if all your employees worked 4-days week doesn't mean your business opens only 4-days week. You simply ensure the amount of workers who are off on Monday

Even if all your employees worked 4-days week doesn't mean your business opens only 4-days week. You simply ensure the amount of workers who are off on Monday is the same as the number of workers that are off on Friday. You only need to increase headcount by 20% if you make all your employees 4-days week. Which is a lot but not a "massive increase", especially you will be paying them 20% less each.