This news has been repeated by many publications and they all say that it raises productivity. I personally wouldn't be so sure. There is a huge effect of novelty here, yeah some startups have adopted a 4-day Week, but I think at Microsoft it is quite new and I can imagine that people get quite excited, I would like to see if the company keeps this and what the productivity will look like in a year.
But don't get me wrong, I am not pessimistic, I am just jealous :)