I work Monday to Thursday and dropping Fridays is one of the best decisions I've ever made. It makes me very happy. I am not interested in dropping any other day and picking Fridays back up. Obviously this costs me financially but I do not care. It's worth it for my mental health.
Would you apply for other jobs requesting Monday to Friday knowing you're going to ask to only do four days? I think it would be a piss take to request this once you've started the job but it's fair enough to ask at the interview table? They'd be saving money after all?