I worked at StreetEasy for 3 years (helping lead the dev team and writing lots of code) when it was a fast growing startup and we didn't work a 4 day week, but we worked normal business hours (9-5,10-6'ish) and never ever worked weekends - unless you felt like working on some code. One of the co-founders there had the opinion that we were in it for the long haul, not for the sprint, so why burn out early. This seemed to work out well [1].

http://www.bloomberg.com/news/2013-08-19/zillow-paying-50-mi...