By the power of truth, I, while living, have conquered the universe.
Karen Traviss is writing a trilogy of novels set post Halo 3, it's a follow up to Ghosts of Onyx and continues the storylines around some of the characters.
Frankie pretty much confirmed at the Halo Universe panel that a Halo 4 will happen at some point, saying they'd be "the world's biggest -blam!-s" if they didn't.