If it were your choice, would you get a brand new RX100 iii or a used RX100 iv. The iv comes with two extra batteries and is about $35 more. Mainly be used for photography with maybe a few digiscoping videos.
If the RX100IV is in great shape then go that route. It's a no brainer. Good luck. If you decide to go new RX100 of course we will take care of you as a Rokslide member
I've had a Sony RX100 III for several years now and love it. Throw it in a Pelican 1010 case and you've got a powerhouse that too small not to take on every single trip.
I'm not a camera expert at all but I was of the understanding that shooting photos, the III and the IV were very similar but the primary difference was that the IV has better video capabilities.
I had an A6000 and opted for the RX100 mk5 only down side is no interchangeable lens how ever the screen flips around so if you plan on doing any video it is great and shoots 4K. I recently purchased a a7iii and still carry the rx100 around as well it's a great little camera