Timothy Williams, a composer that has created music for the recent Conan the Barbarian film, Rise of the Argonauts, and Valve's DOTA 2 has listed God of War 4 as one of the project he's worked on, providing another indication that Sony is working on a new entry in the franchise.
Although Sony hasn't confirmed that God of War 4 is in development rumors and retailer slips have long suggested an announcement is on the way.
In October of last year Sony put the word out for a senior staff concept artist to "Join the GOW team".
A month later New Zealand-based online retailer Mighty Ape listed God of War 4 for release in 2012, giving strength to PSM3's insider information, which said that a fourth entry in the series is set for release around September 2012.
It seems like there's too much evidence to ignore it. Fingers crossed for an announcement soon.