What to ask potential employer / and when to moot 4 day week idea

[MacmillanDo]

I've applied for a job I thought sounded really interesting. I didn't expect to get anywhere, but they've asked me for an initial short call (15 mins) to follow up - for them to ask any questions thrown up by my application and for me to ask any questions I might have about the company or the recruitment process.
I'm guessing it's a quick does your face fit moment.

What sorts of things can I ask that are helpful at this point?

And, really, when I applied I thought if I got anywhere I'd ask if they would be open to a 4 day week (they're a very progressive sounding company).
When is a good time to do this? This would really be my preference. I'm a transparent person, but equally, don't want to blot my copybook too early.

I work four day weeks and always ask in the interview about flexibility and working hours etc. and then I’ll make my formal request after the job has been offered, as part of the standard negotiation around pay etc.

Unless it’s an absolute deal breaker for you, I tend to ask questions around salary and working conditions after I have the role when I have a bit more bargaining power, or you could just get HR stock lines.

I’ve not had this before (unless I proactively ask for it), but seems to me to be a good opportunity for you to express your interest in the role and make an initial good impression, I’d research the company and ask questions specific to the role itself.