Just last week, the latest PlayStation 4K rumor mentioned GOW4, which everyone believed was God of War 4. Yesterday afternoon, now defunct website NerdLeaks posted several pieces of concept art showing Kratos with a beard and an axe. The entire NerdLeaks website has gone offline since the initial posting, but not before NeoGAF users could salvage the work. The pieces originated from the personal website of someone who worked at Santa Monica Studio last year.
Multiple pieces mention Kratos by name and how the environment will interact with him, removing all doubt that anyone else could step up to become the protagonist. According to the images, the God of War icon will be visiting at least two of Norse mythology's Nine Worlds and will at least visit a prison. Industry insider shinobi602 mentioned God of War 4 and Valhalla in a tweet yesterday morning before NerdLeaks posted the concept art. Polygon has also received word from their sources that the pieces are legitimate, so there's a real possibility Kratos will be finding himself matched against Thor or even Odin.
Sony has declined to comment on the authenticity of the images, but that could simply mean they aren't ready to talk about a God of War 4 yet. Until Sony or Santa Monica Studio confirms or denies the existence of God of War 4, all we can do is speculate based on the images shown. If this turns out to be true, Alfheim, Vanaheimr, and who knows how many of the other Nine Worlds will be waiting the destruction of Kratos on his next journey.