4 tens is so much better than 5 eights.

Going to need a citation for that, because that is not what the research I’ve seen suggests.

From this paper:

This paper studies the link between working hours and productivity using daily information on working hours and performance of a sample of call centre agents. We exploit variation in the number of hours worked by the same employee across days and weeks due to central scheduling, enabling us to estimate the effect of working hours on productivity. We find that as the number of hours worked increases, the average handling time for a call increases, meaning that agents become less productive. This result suggests that fatigue can play an important role, even in jobs with mostly part-time workers.

The full article is behind a paywall, but the discussion paper is available in full here.

Another paper suggests that, while same-day performance can sometimes increase with number of hours, next-day performance suffers. And, more importantly, routinely working shifts that are longer than 8 hours reduces productivity over time:

To start, we elucidate the tension between a widely held assumption grounded in the ideal worker norm that working long hours boosts performance and recovery research suggesting the opposite. Our findings reveal that, although working a long day is associated with a same-day performance boost, working long hours hinders overnight recovery by reducing sleep, hindering next-day performance. Our results also cautiously suggest that regularly working longer than 8-h days results in fatigue and a lack of resilience, making it difficult for employees to sustain high levels of performance over time.

I have consistently seen the four-day workweek defined as 32 hours per week (4x8hr), not 4x10hrs.