I'm a bit confused by your comment. By workday, do you mean individual for employees, or do you mean "business days", like when banking and financial transactions are historically run?

4 day work week would be for individual (full time, salaried) employees, to have a 4 day/32 hour workweek instead of a 5 day/40 hour workweek (at the same compensation). Companies like Amazon running 7 days a week just means the business doesn't close down over weekends or such, but doesn't generally mean a given employee is working 7 days straight. (Though, it absolutely can result in employees working 7 days straight, depending on pay period, how weeks are broken down for scheduling and payroll, and whether overtime is allowed and/or encouraged.)