According to respected unofficial PlayStation magazine, PSM3, God of War 4 is currently in development, and is being planned for a September 2012 release.
Rumours started up when a CV (resume) appeared online referencing God of War 4 cinematic testing. This was declined as an official project, but PSM3 claims they have another source who confirmed that God of War 4 was in development, and it will be coming out in 2012.
If this is the case, we might well see something revealed about the game by Sony at this year's E3. Here's hoping, at least.