Considering the cliffhanger at the end of Halo 3, more games are ensured; I see two possible outcomes.
1) This is a big scheme by Bungie. They will either go on to their in-progress IP for a while and go back to Halo on the next generation of gaming platforms, or they may even tie their new IP into Halo sometime. Admittedly, it would be awesome to finish the third in a trilogy of beloved games and find out it's all taken place on the planet Master Chief was near after the credits of Halo 3. Either way, it is fully possible that Bungie has a plan, and will let the franchise lie dormant for a while before picking it up again later in one way or another.
2) Microsoft, holding the right to Halo, will assign it to another company. This is not the preferred option, since franchises have extremely rarely survived a transition from one studio to another (I still mourn Crash Bandicoot, Spyro the Dragon, Star Fox... some would argue even Final Fantasy since Enix merged with Squaresoft). If Halo switches developers, we will have different people doing everything. Halo's gameplay has been ripped off countless times since its creation, so I don't worry for that so much as for... well, everything else. I doubt the story would be as good unless Bioware does it, which they won't. The music would most likely not be by Martin O'Donnel, which saddens me immensely (say what you will about Halo, but no one can talk smack about the brilliant orchestral score).
My preferred option is that Bungie has a plan and Microsoft agrees not to get in the way of it. I'd hate to see Halo get ruined by another developer.