If this approach works startups will leverage it to gain an upper hand. We've seen this with remote work. Startups identified this gap and used it to outcompete other larger companies.

Personally I believe in remote and that it will get even better as tech to facilitate it improves.

I don't believe in working 4 days for engineering specifically. The field changes rapidly, there is a large learning curve and it takes a lot of hours to really master it. There is also some fixed overhead in communicating with other departments which stays the same even if you work less hours. So you remove more than 20% of your productive hours with this approach. You also reduce the time you spend learning. This further hampers your productivity. I think it will be a niche thing only done by some startups that care about this, have no other way to attract talent and aren't very ambitious.

That being said, for some people it will work well. It's pretty common for some roles in the netherlands for instance.