I'm basing it on the book Halo: Cryptum by Greg Bear. It stated that there is only one last Precursor before the Forerunner/Precursor War ended. Plus I believe Sigma 7 (the planet that we see in the Halo 4 trailer) is actually a shield world that The Didact (husband of the Librarian) created so they can protect themselves from any type of danger, making it seem that Forerunners are still alive. Yet there something interesting about the Forerunners. Frank O' Conner said that we are going to fight an ancient evil, and in the book, Cryptum, humans and the prophets fought against the forerunners for freedom (Forerunners were actually control freaks and believed they were the dominant species) therefore can speculate that the last Forerunners will be the enemies we will have to fight.