Austrian 1915 4 ducat mounted on 18k pendant with 20 inch 18k chain.. looking…

You didn’t show a picture of the coin so I can’t really give you a proper evaluation. Your question has an implicit assumption that’s flawed. There’s no reason to assume the numismatic value is any more than the melt value. These are usually modern bullion coins made after the date, and the most you can expect to get out of it about $1,400. And that’s presuming it’s mounted properly, in a proper bezel, without polishing and wear, and not drilled or soldered. The chain itself may add a couple hundred dollars in value.