Thinking about metrics you could be considering when implementing a four day week, a trial or an implementation these spring to my mind:

– Performance (employee)

– Work-life balance

– Stress levels

– Sleep (most companies report 40% better quality sleep and 1/2 hour more sleep per night – on average)

– Sick leave (down up to 60% vs 5 days a week)

– Customer revenue / customer satisfaction (normally no loss, and likely 1.25% improvement)

– Retention / turnover (large improvements, big financial impact)

A few of these metrics you can measure with your company business data (sick leave, revenues etc). And others you should consider measuring via surveys with your teams.