The proof is in a teaser video located underneath the "Credits" option. But in order to access it, you'll need to earn all three platinum trophies across the three games in the HD rerelease package, Joystiq and Kotaku readers have reported. Or you can just watch the video just below provided via a fan with a camcorder:
== TEASER ==As you've noticed, the 30-second video neglects to mention a developer or a release date, which leaves us with some unanswered questions. It's a sure bet, though, that the game will be published by SCE and will appear on, at least, the PS3 since it was teased on the console.
One assumes that the developer of Sly 4 is Sucker Punch. The studio has been responsible for each Sly title to date (excluding this rerelease), and it did sneak in a Sly 4 Easter egg into the original InFamous, which left fans thinking it's behind the wheel of a new Sly title. That said, Sucker Punch is hard at work on a sequel to InFamous, meaning it might not be in a position to do most of the work for this latest Sly--provided that development is ongoing in earnest, which is something we're not sure is happening yet. But, hey, a new Sly! It's been awhile, right? Are you pumped?
UPDATE: A little information added, including the gem that Sony owns the Sly IP.