4-day work weeks have been completely normal for lots of people in Europe for decades. In the Netherlands it's almost the norm to chose whether you want to work 4 or 5 days. 4 day work weeks are not always accepted everywhere, but usually they are in most places.
In France the standard work week has been 35 hours for the past 2 decades, which may or may not equate to a 4 day work week depending on how you organize your work days.
The Netherlands and France are still alive and kicking. They have plenty of successful businesses.
Conclusion: no, 4 day work weeks will not destroy your country, economy, business or lifestyle. The one and only reason to have 5 day or longer work weeks is: greed. IMHO.
Having said that IMO a more important factor in time savings would be remote working or not. Assuming a commute of an hour a day to and from work (2 hours total per day, reality for lots of people) you waste 10 hours per week of your life commuting. Unpaid.
5 days a week without commute = -10 unpaid hours spent on work related things (vs with commute)
4 days a week with commute = -2 unpaid hours spent on work related things.