I feel that the Halo universe has been pretty well fleshed out already, and I don't see them being able to expand on it in any interesting way. Halo 3 was already kind of dragging in the 'new story elements' department. Do you really believe that the Halo universe can support three more storyline game that were as interesting as Halo?
From what I gather, it should be a completely new story line arc, that is set quite some time after the events of the previous games, so you'd get to see how the world has changed in the (hundreds?) of years since you've been gone.
So they're making one...
I guess potential money speaks louder than the words "Halo is a trilogy."
They're doing what they did with the Xbox to 360 transition - making sure they have a 'must buy' sequel to sell the next console with. In this case, it's Halo 5 after being teased with Halo4, much like Halo 3 with respect to Halo 2.