[chrisjharris]

You could see the shift to a four-day week as increasing aggregate demand while decreasing aggregate supply. Prima facie it might seem like fewer hours worked = lower total output = lower GDP, but the macro-economy is a complex system with many endogenous variables (inflation, foreign exchange, debt levels, etc), and the interactions between which can be quite subtle. A lot of schools of economics might put together an argument that this could increase GDP, rather than decrease it. To my mind it seems to increase quality of life which is a more relevant metric than GDP anyhow.