I think these posts are something like productivity porn for life hackers or 4 hour work week people.

I definitely agree that 4 truly productive focused days of works beats 5 lackluster uncreative and unfocused days and I applaud creative work/life balance ideas. But, I think 5 truly productive focused days of works beats 4. As PG says, if you think of a startup as a chance to compress your whole working life to just a few years, you can probably attain that prolonged productivity at least for a few years. Yes, worry about burnout, give yourself time, etc... But one of the things startups do to beat big companies is outproduce them.

Obviously for lifestyle businesses and probably a variety of industries this may not apply. YMMV