I'm pretty sure I'm making more right now compared to how much I was when working full time.
I explain this by having had more time to become a better engineer, as I've been able to spend time on side projects (learning new things), distancing myself from my professional duties (thus avoiding unnecessary/inefficient work), and recovering better during the weekends. Exercising more probably helped a lot too.
Overall, I deeply think that more small tech companies should start offering 4d/week positions, as it will make them able to compete against bigger companies that can offer bigger compensations. Most of the best programmers I know would happily trade a 15% net income reduction for an additional day off. This is also reflected by how popular such articles get on Hacker News or Reddit.
As a side note, part time work for white-collar positions is already very common in the Netherlands[1], and the country if one of the richest in the world.
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[1] https://www.economist.com/the-economist-explains/2015/05/11/...