Here we go again - it wasn't long ago that someone accidentally spilled the beans that he was working with cinematics on God of War IV, which was then quickly denied by himself and by Sony and that it wasn't a game called God of War IV at all and that we should all stop looking at them like that.
But the cat is out of the bag and now PSM3 says that they have on good authority that God of War IV is under development and that it should be out in September next year. The magazine claims that the source is working on the game and that we might even get co-op in it.
That God of War IV would be under development shouldn't come as such a big surprise, really, but it's always nice to get new rumours to speculate on.