We work a 4-day week and just raised $4.75m
The 4-day week is a great idea, and can probably work well in most vertically-integrated businesses.  Ironically, it's probably most suitable for manufacturing work, especially if you're building to stock and not to order.But if you have a lot of external dependencies, and need to interface with suppliers and customers on a regular basis, a 4-day workweek can really into the amount of time you have available to do so.