There are no legal implications that you need to consider other than a reasonable notice term. Best practice for such a transition, depending on Industry and how long you have had a 4-day work week, is 30 days minimum; however, in the hospitality industry that can be as little as 7-days. You may want to consider family obligations and child-care for some employees. It may be difficult, for example, to find child care for infants or toddlers for that fifth day, so as much notice is not only respectful to the employee but might save you some attrition and/or turnover costs.
