Brazilian website, the largest entertainment website in the country, have word from an insider source that the sequel to PS3 title, The Last of Us, is being developed at Naughty Dog, alongside Uncharted 4. The game is being developed by a secondary team and is scheduled for release in 2015.
A secondary Naughty Dog team was also involved in the first game, rather than the team working on Uncharted. That meant that production of The Last of Us overlapped with Uncharted 3, so it very well be the same situation again.
An anonymous source revealed the news, but since they cannot always be trusted, it’s safe to classify this as a rumour for now. It’s unclear what route a sequel will take, especially since the first game worked well as a standalone title.