The Short Version
The claim is directionally supported but overstated. Large-scale pilot programs — including the UK's landmark trial and studies cited by the APA — show that many organizations maintained or improved output on a four-day schedule. However, these results are preliminary, depend on deliberate workflow redesign, apply mainly to knowledge-work sectors, and come from self-selected participants. At least one controlled study found no statistically significant productivity effect. Presenting this as a universal truth omits critical conditions and limitations.