peek productivity not means peek output, i'm agree than the possibility for 30 or 20 hour work week should exist but i'm working 20 hours and even when less productive i will make more in 30 hours, i don't want so i work 20, but i will make substantially more output in 30 hours not 50% more but 35% so my compensation make sense being lower , the 4 day work week means that productivity compensate the loss of hour so the output and to consequence the salary don't change.