no idea... tho that's a very cool place
first... i'd mention that in geology standards, 4000 years is a pretty small measure, so the erosion over that time would probably be all but nothing. for significant noticeable erosion, you'd be talking millions or billions of years, not thousands.
i'd probably go with basically a sunken mountain range, so for the start, take a mt range, and sink it, so just the peaks stick out of the water... The problem with overall shape and erosion patters is it all depends on the world.. there's a lot to think about when you talk about land placement, and there are many factors that effect erosion. limestone is fairly hard i believe, so if it's simply water erosion, then things will go fairly slowly, resulting in the eroded base you see in some of the most iconic pictures from Ha Long Bay... caves and other weathering would happen with differences in erosion times, usually different materials, being a softer rock that would erode faster than the surrounding limestone. salt water is much more erosive than fresh water i believe, so if you're in a salt water sea, expect more erosion.
When it comes down to it, it's fairly hard to have a wrong answer, cause you can back up pretty much anything you'd decide with some sort of geographical example... if not.. you could always blame it on magic.
as is, i think you got quite the pirate haven brewing there, interesting setting to be sure.