Does the four-day week mean
1) the same number of hours distributed over four days instead of five, same work for the same pay?
Or 2) the same hours each day as now but only four days out of five, so less work for less pay?
Or 3) would the pay be the same as before, same pay but for four-fifths of the work?
If 1), it has attractions.
If 2), there would have to be either more people employed to get the same amount done (costing employers in many indirect ways), or there would have to be overtime (again, costing employers more) - which everyone would want to do so as to kepp their income the same.
If 3), employers would have to pay the same but get less done, so they would have to provide overtime (and pay more) or employ more people (and pay more)
In scenarios two and three, all prices would inevitably go up to cover the extra cost.