Posted today on LinkedIn, the new job listing for a Game Designer explains what the position will entail:

Responsible for designing and producing engaging and fun third-person action gameplay and levels

Candidates also need to have “experience playing 3rd person action games”.

Additionally, it’s clear the listing isn’t simply for Uncharted 3: Drake’s Deception DLC, as the designer will be working on singleplayer levels, and Naughty Dog have specifically announced that there won’t be singleplayer DLC:

Responsible for the planning, levellayout, setup and tuning of single-player levels, from high concept to object placement and scripting

The fact that it specifies “third-person action” could mean a new Uncharted, a new IP, or even possibly a new Jak & Daxter. However, exactly 465 days ago, Naughty Dog had a job listing for a game with “fun third-person action gameplay”, and we all know how that turned out, so that does seem to suggest the game could be Uncharted 4.