There is nothing wrong with a four hour work week. The reason people still work long hours is not careful engineering of the economy to keep demand high, but people's natural desire for essentials like healthcare, and also luxuries and status-signalling goods. Also hours seem to be decreasing in Europe, which has a different culture to the US.
Also, one (left-wing) colleague of mine made the following point to me when I said something similar about economic growth: Would I be happy to see the world level out on a 1920's standard of living, including healthcare? If not, why is the current standard of living acceptable? Especially when a given average standard of living will also entail people who are much worse off. So to raise the standard of the worst off, we most likely need growth for everyone.
Instead of trying to increase demand to match supply, what if we decreased supply, like having a four hour workweek?