"Why Spirit?"
Sandy said to us at the interview (granted, it was 3-4 yrs ago) that it's all about QOL because we don't have the best pay.
So use that. You want to live in base and have a good QOL. Play up the 4 day thing, and tell them you care about more than just money.
It's not that hard. I mean, why are you applying if you don't want to work here? And that's your answer, as long as it's not "free type rating."