Welcome aboard, Ruby!
Since you'll be stopping for a wonderful full beach day at Castaway Cay, I'd recommend trying some other adventures in St. Maarten and St. Thomas that aren't just beach days. I promise your Castaway Cay beach day will be the best one anyway, so why not head out to explore the local culture in those other ports?
Every four-year old loves a visit to a Butterfly Farm. There are actually two, one in St. Maarten and one in St. Thomas. Both are great, but can be combined with other tours too, so I'd suggest taking a dual tour that incorporates the butterfly farm and a tour of the island or a stop at another popular destination. In St. Thomas the
Coral World and Butterfly Farm adventure, allowing you to visit both spots, with a bit of time on your own at the beach too!
In St. Maarten, I'd suggest the
Under Two Flags Island Tour if you've already visited the St. Thomas butterfly farm. This tour is great to learn about the two nations that divide this tiny island. While a lot of this will be over your little one's head, the grownups will certainly learn a lot, everyone gets a gorgeous tour, and there is a nice amount of on your own time in two different spots. Your son will love a ride on the Venetian carousel during part of your tour and a maybe even a delicious scoop of gelato! At the end, guests get to choose whether to head back to the port of stay in Philipsburg to explore more on your own, then grab a taxi back to the port. I love this option because you can feel it out as the day goes on!
I hope those suggestions help, Ruby! Have fun planning!