I also do not know of any period or place in time where 2-4 hours of work were sufficient for the majority of people.
So maybe I'm misunderstanding you but why do you think 8 hours is extreme/too much and why 2-4h should be sufficient.
I work in a research environment and it's not like creativity just turns on and off with a switch because it's 8am on Monday and someone said I need to be working right now. It also doesn't turn off at 430PM because that's when work ends. You can require me to be there and have me iterate on existing ideas, but you won't see any profound output, just small iterative improvements. My greatest ideas happen in my leisure or during casual yet in-depth conversations. Heck, some of my best output happens when some idea hits me on a slow weekend morning or late in the evening and I delve deep into it.
For physical labor, 8 hours a day is pretty doable, I'd say 6-7 is reasonable. I've done both types of work and don't have much problem with 8 hours that don't require me to think very deeply. Typical office work fits in here and involves doing simple broad tasks, often mixing in some random light physical activities and usually has little work baggage that you carry home with you (unlike creative work).
If the work is very physically taxing, I feel it has the same effect as very mentally taxing work in that you can sustain it for much less time (say, physically moving heavy objects which requires manual downtime). I recently moved myself and no matter how motivated I was to move those boxes, my body needs a physical recovery break.
The work day used to be 12 (or in some cases more) hours, and a lot of people fought and literally died to get it down to 8. And the expectation wasn't that it would stay there - instead, the common idea was that as productivity due to technological advances etc. would keep pushing it down to only a few hours per day.
Instead, productivity has soared and the parasitic capitalists of the world and their legion of class traitor middle managers have managed to convince people that 8 hours is somehow perfect even though rarely anybody even actually works 8 hours these days - how long is your commute? Do you get work related alerts on your phone? Coworkers/bosses messaging you? That's all time wasted thinking about work. This small group of people - probably what, no more than 5000 out of 7bn people? Are hoarding and stealing obscene amounts of ill gotten gains, killing the planet in the process and ruining probably billions of lives, while we sit here and argue about whether or not we "deserve" to work less or whatever.
Among the results of this increasing productivity is increased consumer choice. Consumer choice is almost always considered a good thing. As consumers, we don’t just argue about whether we all ought to buy X brand of bread or Y brand. Instead, we get a plethora of choices and the individual decides. Shouldn’t we expect this same ballooning of choice for producers? At a glance, it seems asymmetric, and it doesn’t seem like the present consolidation of production towards fewer but larger companies is going to help this.
What would fail to work, in your opinion?
A couple of observations:
First, actually productive work is probably on the order of that anyhow, most of the "8 hours at work" tends to be wasted away.
Second, a lot of our material possessions can now be provided with automation.
So there's probably not that much if any economic loss if people worked less. However, let's assume for a second that working 4 hours instead of 8 would, in fact, reduce GDP by half. In 2017, the US real GDP was $18 trillion. If we halve that, we go back to 1989. We were actually pretty well off in 1989[1].
And yes, macro-economically I also don't understand why recessions are so horrible. Let's assume they throw us back a year. So what? Things were generally pretty OK last year, and at any rate not much different from this year.
AFAICT, the reason recessions are so horrible is that they hit very asymmetrically, so some people are a lot worse off. So it comes back to distribution.