Well, If you happened to see the Legendary ending on Halo 3 after you "Finish the fight", The chief actually doesn't die. So, anything is possible. There is a Halo Wars coming out, though. Be ready for that.
Okay, I've heard a lot about Halo Wars. That game is going to be coming out for SURE. But a Halo 4, would actually be very surprising. As much as I would like a Halo 4, I think bungie has moved on. They might make games based around the main idea of the halo games, like they might make a game about Johnson or something. Instead of Master Cheif. But if so, they probaly wouldn't call it Halo at all. And also, please take a look at the back of a Halo 3 case, and you'll read "The stunning CONCLUSION to the Halo trilogy." Now that doesn't mean that bungie is a hundred percent sure about concluding the Halo series, but there is a good possibility that the series is over. So don't get your hopes up people.