I had the advantage of working on a contract basis, so there was never really any negotiation involved. I informed them that I'd be dropping down to four day weeks, so it followed that my invoices would reflect that. No big deal. Not so much as a word of objection.
There are a few nice things about being in the single best market for developer talent in the history of ever. Being able to prioritise quality of life in ways like this is definitely one of them.