The guys at bungie said that there will be no halo 4 but idk they might come out with it. There is not really much of a plot left for the game though. The scene after the credits was just like a teaser thing like the one at the end of x-men 3.
You did watch the scene after legendary mode correct, it's a tad bit longer. They left it wide open for whatever they want to do.
I think they will make another "halo based" game, maybe they won't call it halo. I think the story line for halo 3 was a little dry compared to 1 & 2. Now that they've finished practically everything that halo 1 started, they can start fresh with something new, but still have the same old halo feel.