Yes. Unless it's deliberately playing games with definitions, a trilogy can only have three parts.
Halo 3: ODST, also known as Halo: Recon, is not the last of the halo series. It may be the last piece of the puzzle involving the Covenant invasion and the search for the Ark. Bungie is planning on dropping a new Halo FPS called Halo: Reach. This part will involve a previously unseen team of Spartans. Follow this link for more info http://halo.wikia.com/wiki/Halo:_Reach
I hope this information satified your hunger.
Knowledge rains supreme overall but delusion of wisdom nurtures the illusion of power & destroys worlds
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