In the UK, it's certainly worth trying. Apart from purely the fact that places may prefer to keep you for 4 days rather than lose you, employees have the right to request part time work (amongst other things). Businesses have to give a reason if they say no, and the reasons get harder to justify typically as a business grows. Not guaranteed, of course, that would be rough on all businesses but it's certainly something that should be taken seriously.

More info: http://www.acas.org.uk/index.aspx?articleid=1616