“Supervisors said staff were more creative, their attendance was better, they were on time, and they didn’t leave early or take long breaks,” Mr. Haar said. “Their actual job performance didn’t change when doing it over four days instead of five.”

Similar experiments in other countries have tested the concept of reducing work hours as a way of improving individual productivity. In Sweden, a trial in the city of Gothenburg mandated a six-hour day, and officials found employees completed the same amount of work or even more. But when France mandated a 35-hour workweek in 2000, businesses complained of reduced competitiveness and increased hiring costs.

[Is the 9-to-5 approach still ideal for workers? We asked the experts]

In Perpetual Guardian’s case, workers said the change motivated them to find ways of increasing their productivity while in the office. Meetings were reduced from two hours to 30 minutes, and employees created signals for their colleagues that they needed time to work without distraction.

“They worked out where they were wasting time and worked smarter, not harder,” Mr. Haar said.

Andrew Barnes, the company’s founder, said he believed his was the first business in the world to pay staff for 40 hours when working 32; other firms have allowed employees to work shorter weeks by compressing the standard 40 hours into fewer days, or allowed people to work part-time for a reduced salary.

Mr. Barnes said he came up with the idea for a four-day workweek after reading a report that suggested people spent less than three hours of their work day productively employed, and another that said distractions at work could have effects on staff akin to losing a night’s sleep or smoking marijuana.