Looks Like Guerrilla Might Be Taking A Break From Killzone After Three's Release.

Their priority is currently Killzone 3, but Hulst has revealed that the studio are currently at the concept stage for the new property.

It's assumed that Guerrilla will ramp up development of the new IP once Killzone 3 is complete. No further details were given. You probably shouldn't expect any until this time next year.