Kratos may be fitting his gauntlets for a new madcap murder spree next year, according to the unofficial PlayStation magazine PSM3 (via CVG). The magazine claims that it will be coming in September 2012, which would break from the series tradition of spring releases. CVG also cites unverified rumors that the next installment will include online co-op.
Kratos' fate in God of War 3 was ambiguous, but left serious hints that the character lives on in come capacity. The game was released in March of 2010, so an autumn 2012 release seems reasonable. If true, this could be announced at Sony's E3 presentation this year. As usual, consider this a rumor until we hear it from Sony's mouth.
In the meantime, try to figure out what he'll take on next. Kratos has pretty much wiped out all of the Olympians and Titans by this point, so who's left to brutally murder? Maybe he'll move on to other arcane religions and start wiping out large swaths of Valhalla.