Maybe! Apparently the stealth game was accidentally revealed on the PlayStation.blog, but then taken down. There were apparently discussions of this on the PlayStation Community Forums, but they too were taken down.
According to a post on GameTrailers.com, the game is going to be a PS3 title and will be out in late 2012.
Sony told Eurogamer that it would not comment on "rumor and speculation".
Expect more info at E3 about Syphon Filter 4 - if true. Dun dun dun.
Rumour: Syphon Filter 4 Revealed on PlayStation.Com [Giant Bomb via Eurogamer]