"I can be true about it, yes. That's definitely what's happening right now," Boltjes said. "As a studio we do want to branch out, and we have started work on a new IP, something completely different to Killzone. I don't want to say anything about it right now, but as a studio we do want to keep it fresh." Unfortunately, that's the extent of what Boltjes offered regarding this new IP.
It's common practice for Guerrilla Games to begin work on its next game before it's shipped its current project. Guerrilla Games began development on Killzone: Shadow Fall before it had even shipped Killzone 3 back in 2011.