Croatia and Bosnia-Herzegovina in October?

Hmmmm. Thoughts:

You could easily stay in Greece and see more than just Athens.

Then I would check flights and look at what is an easy option. Would you be able to fly home from your second destination or would you have to go back to Athens? If able to do multi-city, look at how you can get home from there.

If you have 4 days, the first will be getting to your second destination. So then I assume you have 3 days. Dubrovnik will probably be less crowded than summer but 3 days is just about right. I absolutely love both Bosnia and Montenegro, but I would probably opt to stay put and see what Dubrovnik offers, including a day out to Lokrum, although I know plenty of people are willing to spend half a day in a car or bus.

If you want to see Bosnia , fly to Sarajevo and spend your days there. I can give you a great company who does small group tours / but the city itself is so interesting.

Kind of like going to an all-you-can-eat buffet, but after looking, you can
only choose one item to enjoy....

I think "less quiet" in Dubrovnik might be a relative concept. Yes, less
crowded, but definitely not empty. I was there beg of May, so not quite
inverse of your times, and the place was very crowded by my standards.

I assume you have your reasons for considering Dubrovnik, so leaving it
at that, I think 2 full days is enough to see the core sights. It's not clear
if your 4th day is a sightseeing day or a travel day; if it's the former, the
time for a day trip certainly might exist.

DBV does not have all that many flights, so to reinforce TTM's comments,
your flight schedule and costs (no matter if you're paying or your business
is paying) are worth researching upfront.

Direct answer: Mostar. Indirect answer: Fly Athens to Podgorica, spend all the time in Montenegro. Maybe one night in Dubrovnik before you fly home.