As reported by Eurogamer, it seems Crytek itself may have leaked Crysis 4 before the game could be officially confirmed.
Crytek China recently took to Chinese social media site Bilibili, and in a post that has since been removed, seemed to confirm that Crysis 4 is in development.
Many had hoped that this recent activity means Crytek might be gearing up for a full-fledged new Crysis game in the future- and it seems that might indeed be happening.
In the absence of an official announcement, for now we’re going to have to treat this as a rumour.