Something I've never understood is why four days? Is it simply because it's an incremental improvement over the current five day week, a stopping point toward the next target, or is it the target?
4 to 3 is a 25% cut and a 33% increase in weekends (3 to 4).
3 to 2 is a 33% cut and a 25% increase in weekends (4 to 5).
Thus we see, 3 work days a week is mathematically optimal.
3 work days might work as well, but fewer than that and you start to lose the flow at work. And given that I do generally enjoy my work, it isn't necessary for that holiday feel anyway.
I also used to work shifts for quite some time, and while I hated working nights/evenings, we "only" worked around 30 hours a week. So most of my weeks had odd days off here and there. Sometimes longer and more consecutive days. That is what I miss the most.
But having tried that, I can say that for me: Three consecutive days off > Two consecutive days off + one random day off here and there > Two consecutive days off.
I ended up leaving myself notes on what I was working on before I left on Thursday and what to do first. This was a big help in getting the continuity back and not feeling like I missed a beat when I resumed working again on Tuesday.
Potentially there isn't a real reason, but that it's just an easier bite to chew for now, where people have noticeable improvements.