I also only applied to four thinking that that was normal! Today marks the end of my involvement in the application process, though - by which I mean no more interviews or writing or really anything else I can do. Just waiting for results now!
I heard from them a few hours after I posted, but I wasn't told who's on the interviewing committee or how many there'd be - I think the first poster was perhaps invited in a different email/method? Maybe informally?