I used to work in a profession where we'd regularly get 100s of applications for 1 spot, and usually preliminary interviews were between 10-15 people. Maybe 8, if you were being particularly stringent. But then there was a callback of (usually) 3, so obviously that's different from here. The problem with interviewing 4 people for 1 slot isn't that you'd have to go past your 4th choice to get someone to accept the offer - it's that you might not end up liking the 4 people you pick (since after a certain point, if you're picking 4 people out of 100s or 1000s, you're really relying on fairly arbitrary stuff to distinguish between them, and someone may look way better on paper than they do in person). I've regularly seen, say, 12 people who looked great on paper get a preliminary interview (like a screener), and after the interview 6 people have managed to rule themselves out without any work on the interviewer's part. Yes, everyone's qualified on paper, but it's not just a question of ruling out the weirdos - you also want to find the person who is the right fit for the slot (and I think this would matter a little bit more in this context where you're hiring one person who's in theory going to be around for a long time, than when, say, you're hiring a big SA class for a big firm, where you have to expect a reasonable amount of attrition). So I could see wanting to interview a few more than 4 for 1 slot - but am just speculating, of course. I don't think it would be more than 8-10, though. (Again, speculation.)
It does make sense, though (as much it depresses me), that a lot of people get designated finalists.