Uncharted 4 will feature new multiplayer content, less linear environments, and the introduction of a new Naughty Dog AI Companion.
Remember when the Uncharted movie was announced, and it was about the entire Drake family instead of just Nathan? It turns out that Uncharted 4's story is taking a slight step in that direction. According to a recent preview, the upcoming Uncharted sequel introduces Sam Drake - Nathan's long-lost brother - who will be an AI companion much like Ellie was in The Last of Us. Except instead of growing to love the AI character, it sounds like Naughty Dog will take the opposite approach by using Sam to create dramatic tension.
Set three years after the trilogy ended, Uncharted 4 opens with Nathan and Elena living together instead of breaking up between games. That's also when Sam returns, seemingly from the dead, and convinces Nathan to return to an adventuring lifestyle. Their goal: Libertalia, a mythical colony in Madagascar founded by pirates. The twist is that another pair of treasure hunters have the same plans, prompting a race to see who can reach the fabled location first.
Like Naughty Dog's last game, Sam will be controlled by the AI over the course of the story. "AI companions" is a phrase that rankles many fans, but in Uncharted 4's case that may be the point. Nathan will clearly be a far more capable treasure hunter than Sam, prompting a slow increase of jealously and recklessness that defines the plot of the game.
The preview goes into detail about other features, like less linear environments, clues for artifact locations, and the return of Uncharted's multiplayer mode. But if you're a fan of previous Uncharted games, the story is probably what you're interested in, and this one sounds like it goes into un... familiar territory.