Why would retention change if everyone works 4 days a week? At that point it is no longer a perk, you get it everywhere, so retention should be as before and the advantages of 4 day weeks is now gone.
I am a little baffled by the idea that job satisfaction is comparative. Three days a week to do what one wants and needs to do is life enhancing for me, even if everyone else has it and retention is a function of happiness.
Because people that are happy at their job tend to stay. Your point is "so what if they are miserable right now? They will be miserable elsewhere, too". That might be, but people will still try to get away from you, hoping it is better elsewhere.
With that argument retention would already be great with 5 day workweek since it is so much better than 6, but you don't see that, retention is really horrible today.