It's a great essay, and Bertie makes some good points. I think the weak spot is that if you somehow manage to get work shifts down to four hours a day, a lot of people will choose to work two jobs. Not all of us have been brainwashed by elites into worshipping the God of Toil; some of us actually derive a bit of satisfaction from work, value leisure more when it comes in limited quantities, and want to buy more goodies. It's great to pursue hobbies and art, but most of the improvement in standard of living for rich and poor alike has come from industry (literal and figurative).
It's great when someone can earn their means to basic sustenance in four hours a day. They can fly kites or write novels the rest of the day, or sit in a cabin by the pond. If they aspire to more than basic sustenance, I say they should be allowed to work as hard as they like. Perhaps they don't love their work, but they love the idea of saving up for big vacations, or having savings to handle emergencies, or practicing philanthropy. Russell, born into aristocracy, didn't have to worry about sustenance and had the luxury of working hard for causes he believed in. Like him, I hope that life will continue to get better for workers. And I suppose life in the developed world today is as much better than it was in 1932 as it was then compared to the times when "fifteen hours was the ordinary day's work for a man; children sometimes did as much, and very commonly did twelve hours a day."