First Round Interview Help- Strange Situation

A few months ago, I applied a non-IBD MO summer intern position at a major bank. I never heard anything back, so I completely forgot that I submitted the application. Fast forward to now, when I randomly check my email to see that I've been selected for a first round interview, to my shock, in investment banking. I'm not sure how I was selected for this, since I had no desire to actually intern in IB, but I have no clue how to prepare for this. I'm also a from a very non-target school, non-finance major, and have done no networking at this bank. I do have a previous internship in financial services, but the work doesn't relate to anything dealing with investing.

Since I didn't think I was under consideration for an IBD role, I have done 0 prep for this kind of interview. I have taken a basic finance class and a basic accounting class, but don't remember all that much from either. I have a 30 minute phone interview with 2 associates and a VP in 4 days. At this point, my main goal is just to not make a fool out of myself, since I can't learn everything in such a short time. What should I absolutely be able to understand and talk about? Also, what do I say if I don't know the answer to a technical question?

tl; dr: Accidentally applied for an IBD SA role without realizing it, but now have a first round interview in 4 days and haven't prepped at all. Need urgent help.