I'm in a similar boat. Depending on your programs, They may hand out invites up until January. I would check last year's application threads to see when people stopped getting invites so you can set your expectations there.
4 interviews is great! As long as you interview well and try your best your chances are very good to get into at least one program.
Depends if the programs are on rolling admissions or not to answer your first 2 questions. It's always worth it to try, but if you're tight on money I wouldn't blow hundreds in a last ditch effort. There's probably a reason you didn't apply to them in the first place!
Just stay positive! You're already ahead of the majority to get interviews and waitlist seats.
I was just waitlisted at my top choice program after an interview. I have yet to hear back from another interview, and have two coming up this month. I am getting nervous that I should have applied to more programs. Is it likely that programs with November/December deadlines still hand out interviews? Would it be a waste of money to apply? Can they see that I was verified on CASPA in July for other programs? Thank you