Not sure what they tell you when recruiting but there's been a big drive lately across the firm to improve flexible working. I know a good few people working 4 days a week at the moment.
Admittedly, most I can think of are either returning mothers or those who have been off on illness, though in principle I don't see why it wouldn't apply to other staff (I suppose those who've had a change in life circumstances are those more likely to ask for a change to working hours).
That said, not sure how it would sit in terms of applying. Again, in principle you're perfectly entitled to ask but all it takes is one person living in the dark ages to say no and you're scuppered.