I'm the other co-founder of Treehouse. It's something that Ryan enacted back in the Carsonified days, but it's worked amazingly well and is just something we've always done.
Company folklore has it that Gill, Ryan's wife, basically commented that it seemed like the company was running them rather than them running the company back then, so their solution was to switch to working 4 days per week.
I'm curious how you guys decided on a 4-day week? Was it a group decision or founder laying down the new way of getting stuff done?