This is definitely a report of two halves.
First up, the latest physical edition of EDGE Magazine [via Gamerzines] states that "the bulk of Guerrilla's staff are soldiering on with the next Killzone instalment," with veteran Killzone senior producer Steven Ter Heide at the helm. It's been described as a "follow-up." This is a big claim, but it's likely that EDGE know what they're talking about - otherwise, they'd have ran the story as an uncorroborated rumour.
Which is where act two begins. See, EDGE also recently ran a story from an anonymous source who suggested that an internal Sony Studio had started developing for the PlayStation 3's successor (the PS4, if you will). So... yeah. You can see where where we're going with this.
Either way, we'll keep an eye on the situation.