There seems an unnecessary presumption that this means 4/5th of the work hours, with an implication of 4/5ths of the output. But a 4 day week can also be four

There seems an unnecessary presumption that this means 4/5th of the work hours, with an implication of 4/5ths of the output. But a 4 day week can also be four 9 hour days. So long as management adjusts daily expectations to match, there's no need to expect lower productivity, or to demand a salary cut. Overall though it's a fairly open secret that, especially in big companies, you can set a 40hr work week, but you're not getting 40hrs of actual work from them. And for some jobs this is just beyond people's fatigue limits anyway. To hazard a guess, for in-office, the average would likely be around 25 to 30 hours. The rest goes into the ping pong table, chatting, morning coffee, reading HN etc. So there's plenty of margin to lower scheduled hours and raise expectations during those hours. There's also plenty of room to give 3 days of rest in exchange for 4 days all-hands-on-deck.