If, as current rumours are suggesting, Sony are aiming for next Christmas for the PlayStation 4 (with a mid 2013 reveal) then it’s likely that their mainline first party studios are working hard on a series of launch (window) titles.
That would include Guerrilla, Sucker Punch, Polyphony and Evolution, among others.
So, let’s speculate on what those teams might be working on…
Killzone 4 – Guerrilla Games
Back in late 2011, Edge magazine pointed to the next installment of the Killzone series, citing Guerrilla’s Steven Ter Heide as the game’s director.
A previous statement from the same magazine also highlighted that “one major Sony-owned studio has now ceased PlayStation 3 development, its entire focus having shifted to the console’s successor” – it’s not guaranteed that this was Guerrilla, but seeing as how this was last November, there’s a fair change that the Dutch studio is indeed working hard on Killzone 4. 
Guerrilla’s last release was Killzone 3, back in early 2011. That would leave them with well over two years development time for the next game in the series, closer to two and a half should the PS4 end up launching in late 2013.
Elsewhere, Guerrilla are working on Killzone Mercenary for the PlayStation Vita, but it’s assumed that it’s mainly the guys at Cambridge Studio here in the UK that are handling duties on that – a series of job advertisements would at least partly confirm my suspicions there.