The sad, revenge-fueled story of Kratos apparently came to an end with the release of Sony Santa Monica's God of War 3 last year. While there's little doubt that we'll see more of the ashy Spartan warrior at some point in the future, the studio has apparently shifted its efforts to focus on something entirely new.
A Gamasutra job posting reveals that Sony Santa Monica is working on an "unannounced new IP." The open position is for a Senior Level Designer, and the listing further notes that candidates with experience developing an open world game are preferred. The God of War series is known for its very linear structure, so it seems that this new game will be a departure on more than one level.