I might get this job, but I have a vacation planned?

I applied and interviewed at a retail store last week. I'm not sure I'll even get the job, but if I do, how can I tell them I'm going on vacation? I said I could start immediately when I applied online last week, but my mom and her new boyfriend invited me on a 4-day vacation. I plan on calling tomorrow to see if my interviewer is there so I can tell her that I need 4 days off. I don't want this to influence her decision, though. Should I call tomorrow or wait and see if I actually get the job (which will happen in the next few days)? My mom's bf needs to make reservations very soon, though, so I'd rather not wait.