A four-day workweek is, ideally, a 32-hour working week with no loss in productivity, pay, or benefits. Depending on the company and the industry, staff might work Monday through Thursday and have Fridays off. Other possibilities include allowing each employee to choose their extra day off or having a company-wide policy of a different third day off, such as Monday or Wednesday.
Naturally, each business is different and whether a four-day working week is a feasible working pattern for all or some of your workforce is entirely dependent on the job at hand and whether sufficient staff coverage is possible.