An italian news site dedicated principally to videogames and technology just posted about Killzone 4 already in development.
http://www.4news.it/9728-killzone-4-confermato-lo-sviluppo-sul-prossimo-numero-del-magazine-edge.php

Endge apparently has announced that in the next magazine it will publish a short article about a new Killzone already in development. Seems like the few Helghast remained have found a new charismatic leader that will counter-attack Vecta...just like a Killzone 1 Remix. It should be out in 2012.

I tried to search on wikipedia but found nothing, then I clicked on the Guerrila Games page and saw the game list. They are developing 2 new games : New IP (PS3) and Killzone 4, on A FOURTH PLAYSTATION...Both TBA and none of the games or the PS4 has a wikipedia page...so no info yet...

Check it yourself Guerrilla Games - Wikipedia, the free encyclopedia