Sly 4 is real. Sly Cooper Thieves in Time was just revealed at the Sony Press Conference. Although we only got a cinematic trailer, I think that's all we need to get Sly fans excited. There's a full demo at E3 and from what I've heard it looks really good.
Sanzaru Games is handling development for Sly 4. sanzaru made Ninja Reflex for the DS and Wii and they also ported Secret Agent Clank to the PS2. They say that Sly Cooper is one of their favorite franchises so let's hope they do the series justice with this new installment.