Sure, I will get more done in five than four days. I also get more done in six or seven days per week over five, but with diminishing returns and more and more impact on my non-work life.

As a programmer I have the luxury of willingly taking a pay cut to have an additional day off per week if I want to and usually in a position to negotiate for it.

I don't know the Finnish economical situation well enough of how feasible a four day week would be short term. But I don't see much difference to reductions of work hours/days in the past.