Myself, I don't think there will be an actual "Halo 4". At least not titled as such. As you said, Halos are deactivated, no longer a threat, neutralized. That kind of takes the power, and indeed, the REASON out of the very title.
Of course, you could start a whole new series starring Master Chief and just name it "Master Chief", but it would mainly be shamelessly milking the series. The Covenant and Flood to my knowledge are all taken care of, so any new threats that arise will be invented almost on the spot. And seeing how I believe Halo is based off a series of novels, this would equate to the pinnacle of ****tiness I like to refer to as "Dragonball GT Syndrome".
It's like any series that decides arrogantly that it's outgrown it's literary roots and decides to meander off on stupid directions, just for the sake of not facing the inevitability that eventually you have to stop moving. Jurassic Park did this as well. Two novels, transferred to two fantastic films, and then because the writer was smart enough to know it was finished, and Hollywood was thick and arrogant enough to think they could pull off the genius of the original purely through screenplays, a 3rd was made totally independent of the book, and guess what? It sucked flaming Satan ****.
I know games have been know to be able to recover themselves after failures, but Halo 3's storyline was lackluster already, even having the novels behind it.
What I do know however is that there were Halo novels that came before Halo 1, as well as between Halo 1 and 2, so it seems more than possible that these could be adapted. Just don't expect it to be labelled "4".
Don't worry though; Halo is as much an icon to the XBox consoles as Mario is to Nintendo, so I doubt Microsoft, and therefore Bungie, will let helmet-head's legacy die anytime soon. If things get too dicy, they'll probably go for broke and just remake Halo with the current graphics for ****s and giggles. Though of course, if Valve, as great as they are, could **** up the improved rerelease of their original, I have no doubt in my mind that Bungie would **** it up similarly. If no, well then congratulations to Bungie for finally doing something better than Valve besides graphics...
Oh wait... ^_^
