There will most definetly be a halo 4. Now, it might not be called that directly, but Frank O' Connor has made the move from bungie over to Microsoft studio's to oversee Gearboxes work on the new title. Also, Bungie has hinted on their website enough that it is in the works. Like all developers, Bungie finished the third game, and probably out of pure exhaustion claimed this was the be all to end all for them and halo. Kind of like you might do after a long night of drinking, when you wake up with a nasty hangover and shout "never again". But just like that, bungie probably sobered up half a year later, and realized that they actually enjoy working on halo games. It's an inevitable possibility. The guy's at Bungie actively support a thriving community, all of whom want another halo game. It would take an amazing amount of willpower to ignore the cries of your fan's, and microsofts money.
That being said, who said it had to be a direct continuation of the story. I always hoped halo would shed the petty diatribes and "Story", in favour of a more pure multiplayer experience. Massive maps, pile's of vehicles, large servers, an armoury of alternative armour types, and an in depth stat tracker that mirrors the way CoD4 had me coming back for more, without actually just ripping them off.