Although some companies have found four day weeks to be successful in productivity levels, other companies have found that a four day week has a negative impact. Treehouse, a company that offers virtual coding classes, found that there was a drop in work ethic and therefore discontinued the policy. 
Some business leaders, such as Amy Finlay, of Edinburgh IFA, suggest that a four day week is unrealistic. Where businesses have small teams, they already struggle to get things done within the five working days, the four working day week would overwhelm many. Ryan Arrindell of Animate Designs argues that it would be hard to have shorter working hours work with the demands for longer opening hours. Bigger companies may be able to cope with this due to their workforce, but smaller ones would not be able to. 
It is also contended that people may feel more refreshed with an additional day off, but productivity may drop due to working a longer day in a four day week. Where you have a mixture of people working four days and five days, or people on alternate four day shifts, someone on their day off may feel more pressure to respond to e-mails or attend meetings, which may overall reduce productivity.