Right, the point of the 4 day work week is that it will become the new standard for full time work, rather than the current 5 days.
So all your points are kind of moot, as they will ideally be addressed through cultural changes, employee expectations, or regulation.
In this case, I would check out the Floorp browser. It is a Firefox fork that plans to be more like Vivaldi and have lots of features, including vertical tabs.