The test took place in Iceland, and involved more than one percent of the entire population - which, actually, isn't really that many people considering the population of Iceland was recorded as 356,991 in 2019.
Even though Iceland has a relatively low population, it's still the biggest study of this kind ever undertaken, and the results are definitely giving countries with much larger populations something to think about.
The scheme saw workers reducing their working hours over the week down to just 35 or 36 hours, with no reduction in the amount they were getting paid.
The results of the pilot, which ran between 2015 and 2019, were then analysed by research teams from both Iceland and the UK.
They discovered that the more than 2,500 people who took part saw their productivity and - more importantly - wellbeing increased.
More permanent changes are already being considered over in Iceland.
Icelandic trade union federations, which are responsible for negotiating the wages and working conditions for most workers in the country, have already started to negotiate for less working hours following the scheme.
The researchers behind the scheme reckon that the new agreements forged between 2019 and 2021 have given 86 percent of Icelandic workers the opportunity to work less hours, or have the flexibility to work less hours.
The trials were conducted by the Reykjavik city council and the national government of Iceland after pressure from unions and civilian groups.
Workers who are on 9-5 working hours as well as those with non-standard shift patterns were included.
The resulting analysis from think-tanks Autonomy in the UK and the Association for Sustainability and Democracy (Alda) from Iceland discovered that workers' wellbeing improved drastically across many different indicators.
Work/life balance and stress were reduced, as well as perceived 'burnout' from working.
Because of that, researchers say, productivity and service provision remained the same or improved across the majority of workplaces.
Alda researcher Gudmundur D. Haraldsson said: "The Icelandic shorter working week journey tells us that not only is it possible to work less in modern times, but that progressive change is possible too.
"Our roadmap to a shorter working week in the public sector should be of interest to anyone who wishes to see working hours reduced."
Will Strong, Autonomy's director of research, added: "This study shows that the world's largest ever trial of a shorter working week in the public sector was by all measures an overwhelming success. It shows that the public sector is ripe for being a pioneer of shorter working weeks - and lessons can be learned for other governments.
"Iceland has taken a big step towards the four-day working week, providing a great real-life example for local councils and those in the UK public sector considering implementing it here in the UK."