The pirate utopia discovered in Uncharted 4: A Thief's End is Libertalia. In the game's fiction, it was a hidden settlement founded by the pirate Henry Avery and other notorious captains.
Nathan and Sam Drake follow clues connected to Avery's enormous lost treasure. Their search eventually leads them to Madagascar, where they uncover the remains of Libertalia. The settlement was designed as a pirate refuge, but its history is marked by conflict, betrayal, and competition over Avery's wealth.
Libertalia is inspired by the broader pirate-utopia legend of Libertalia, a supposedly egalitarian colony associated with pirates in Madagascar. Historians generally treat the historical account as uncertain or possibly fictional. Uncharted 4 adapts that idea freely, giving the settlement a different founder and a central role in its treasure plot.
The wrong options refer to other legendary places. Shambhala is central to Uncharted 2, El Dorado drives the first game, and Atlantis is not the pirate settlement in A Thief's End.