I know that's the popular cynical view, but I'd think with such a position (especially given that it was advertised as reduced hours), there would be 20% lower expectations as to what you'll accomplish. In other words, they'd expect it would take five weeks for you to do what someone else can do in four.

I mean, that's all they have to determine/enforce how much you time you put in currently... general expectations.

Some people say you work the hours needed to get the job done, but that doesn't really make sense. Somehow the work the company needs to get accomplished gets divided up and distributed to employees. And ultimately you've got to think of how much is reasonable to accomplish. Here, that expectation of reasonableness is going to get reduced by about 20% if they are hired to work that many less hours.