I love the line of thinking here, but please don't try this with you're pre-revenue cash burning startup. Best wait till you have solid profits, plenty of users and a nice 'moat' around your business as Warren Buffet would say.
If the point is that you can be as productive working a four day week with a 3 day break I would ask why work a five day week?
Startups need productivity not man hours. If you're happy you might make a 'happier' product, be more likely to succeed with business partners, more charming to angels and VC's, more friendly with your friends and even a better lover with your lover.
Each person only has so much to give.. you can squeeze a lot at the beginning and loose later or take a steady pace and reach the goal with a lot less stress.
But companies are the only ones married to a five day workweek to begin with. The only reason you work five weekdays is to seem professional.
At a startup you should work whenever you feel like it (which is hopefully seven days a week starting at 11pm and going til dawn, with copious amounts of caffeine and loud music). But if you can get a lot done in four days and it'll keep you happy, work four days, you don't have any corporate culture to prove anything to.
The grandparent comment gives my definition of the difference between a startup and a company:
Startup: "...pre-revenue cash burning startup."
Business/company: "...solid profits, plenty of users..."
By this definition, most YC companies would be startups while 37signals/Fog Creek/Google would be companies. Incidentally, Facebook is still a startup by this definition, despite its size.
So like you say, the startup has to do whatever possible, because as I say, they're on borrowed time. They onus is on them to demonstrate that their idea and execution is accepted by the market ("Make something people want"). In a business with customers, profits, etc, they've done that and it's just a matter or managing what has been created. When you get to that point (especially when hiring non-founders), you have room to be flexible.
