Fair warning, this rumor is all sorts of unfounded. An image is going around online today showing off what looks to be a retail poster for Sly 5: Master of Thieves for PlayStation 4 and PS Vita. It's completely insane that a retail poster for an unannounced game, for a console months away from release, would be discovered anywhere. Still, just look at the thing. It looks surprisingly legitimate.
Sly Cooper: Thieves in Time was made by Sanzaru Games and was released earlier this year. The previous Sly game, Sly 3: Honor Among Thieves, was made by current Infamous developer Sucker Punch. Basically all I'm saying is it doesn't really make sense for another Sly game to be anywhere near completion right now. Even if Sanzaru started working on Sly 5 right after Sly 4 was finished, it'd be at least a year, probably two before release.
Basically, even though the poster looks great and virtually every aspect of it seems believable, it's completely ridiculous to think we'll be seeing Sly 5 anytime soon. For now, it's best we just forget this poster ever happened. Go play Sly 4 instead, it's outstanding and you probably missed it!