There will be a God of War IV. There's no way Sony will stop their 2nd biggest, most beloved and most acclaimed franchise. They will find a way to continue the franchise forward and Kratos will not be replaced as the protagonist but he will be very different, with the ending of God of War III it is obvious he won't be the same mentally broken and vengeful demigod, he found the ability to forgive himself, killed everyone who wronged him and sacrificed all of his powers for the sake of humanity. Expect his character to be a more grounded, calmer and a lot more humanistic going forward in the series.  

There will be a God of War IV. There's no way Sony will stop their 2nd biggest, most beloved and most acclaimed franchise. They will find a way to continue the franchise forward and Kratos will not be replaced as the protagonist but he will be very different, with the ending of God of War III it is obvious he won't be the same mentally broken and vengeful demigod, he found the ability to forgive himself, killed everyone who wronged him and sacrificed all of his powers for the sake of humanity. Expect his character to be a more grounded, calmer and a lot more humanistic going forward in the series.  

God of War IV doesn't have to move into diferent Mythologies as there are beings far more powerful than the Gods were, but if they do I hope it's either Norse or Egyptian Mythology. I prefer Egyptian Mythology myself, as does the fan who created this beautiful art work:

It would be great if ill-informed people stopped saying 'they should create a new IP' because we already know that they are! Stig Asmussen is directing a new IP made internally by Sony Santa Monica http://youtu.be/_JxWVtf-xaA?t=17m54s, not to mention that Sony Santa Monica have helped with so many games this generation that wouldn't have happened without them http://en.wikipedia.org/wiki/SCE_Santa_Monica_Studio . 

We shouldn't be talking about God of War IV for now though, everyone should know that God of War: Ascension is DEFINITELY going to have the best single player campaign in the series, we haven't seen much of it yet but I've done some digging and spoken with the developers on Twitter/ Facebook and some of the voice actors and they've been giving subtle hints and pretty much inadvertently confirmed the informantion I have discovered. I won't say anything because it will ruin the story for you but it is going to be absolutely amazing.

Also God of War: Ascension is going to be the best in the seires because it has the deep and personal story from God of War, has the variation of enviroments and sense of adventure from God of War 2 and the beautiful graphics, scale, cinematography, boss fights and gameplay from God of War 3 all combined into one game and then some with new mechanics, new enemies, new settings, perfected gameplay, the best puzzles, improved platforming and of course the genre defining and innovative multiplayer component that will blow everyone away once they've shown all the modes, maps, how extensive the customisation is and how the MP ties in with the single player story.