4 days can meen different things
Seems like both the original article(which isn't totally clear about it, and probably should be) and the survey question(which is inexcusable) weren't clear if they meant 4x10 or whatever that preserved total worked, or cutting a workday without adjusting hours(and with or without adjusting pay).
The impacts from those different plans diverge pretty quick. That said, different people and different industries may be able to make any of them work. I've dealt with the 4x10 flavors (4x10.5-11 actually worked due to lunch, unpaid) which can be a fit for parts of our industry that still need to keep a schedule. Since I currently have to cover most of the working hours of the others in the office for support, and can't work on most of the systems I run till after hours, working a 4x10 when the office is on a 5x8 cuts the overtime to manageable levels, as those 2 hour windows will be enough to take care of most of the routine stuff. But I get paid overtime for over 8 hours, so it was cheaper for them to shift my schedule to just start later. Cutting my hours wouldn't work in a way they would like, and I'd end up working for whatever company I started consulting for to make up for the lost income because I don't like bouncing rent checks.
As for productivity, I'm fine dealing with my current workload, but when I was coding or doing more project related stuff, acute vegebrain was a real problem by end of shift on days when I wasn't in the flow, especially during crunch time. So after 4-5 weeks at 10-12 hour shifts five or more days a week, I was a drooling idiot, and barely able to do what I did in day in a normal 40 hour week. Burnout is a real problem, as is the pileup of real life issues when all daylight hours are work hours.
Can I squeeze the same amount of productivity into a 32 hour week? Depends on what I am doing, most times, probably not consistently. YMMV. Also, many of us don't NEED to get a full weeks work done every week, odd as that is to say. If you arer IT/help desk at a smaller company, you may run out of real work to do sometimes, and need be covering the phones/email/tickets/website/slack/pagers/pigeons in case something blows up or your one of your users are eating paste again. So if you have enough of a bullpen to cover the hours, there are plenty of people that may thrive on a shorter schedule and more free time. There are also people that would get screwed by potentially losing their benefits, a sizable chunk of their income, and be pressured to track and respond to proablems on your "off" hours.
I'd rather fight for better rights for individual employees to seek a schedule that works for both parties and isn't set in stone, but that's not what is flying around. I suspect all this chatter is that the beancounter are recession spooked and looking for a way to cut payroll without giving up anything. Cut their hours on paper, cut their pay, and don't adjust their deadlines or deliverables. Then ride them to make up the difference off the clock or get put on a "performance plan" and use that to fire anyone that cries "Sweatshop! Wage theft!".
Probably won't get hit with that myself unless I move to a less favorable jurisdiction. Due to years of abuse it's now explicitly illegal to not pay overtime over 8 hours, 40 in a week, and a few edge cases. Unpaid on-call status is still a minefield, but more so for the employer so it's also rare in our industry out here these days.