<UPDATE: Sony officially God of War: Ascension on the PlayStation Blog.>
It seems that the announcement planned for today, the 19th April, is indeed for the next God of War game, which is apparently called God of War: Ascension.
Amazon.com leaked the box art, and the trailer was prematurely released on YouTube as well. The site also lists a placeholder date of 31st December 2013.
According to the trailer, "Many know the legend of Kratos, the mortal turned god who brought down the very walls of Olympus, but there was a time before Kratos became the monster known as the Ghost of Sparta. A time when something other than rage consumed him, a time when in spite of the madness that plagued him, Kratos would break his blood oath to an unjust god, a time when he would no longer be bound in blood."
This seems to indicate that the game will be a prequel, which would explain why it's not titled God of War IV.