ODST was two discs because the Matchmaking disc contained all of the Halo 3 multiplayer maps and more. Halo 3: ODST’s main disc engine is slightly modified from the Halo 3 engine, so that’s two reasons why they were two discs. It wasn’t necessarily because ODST’s campaign was longer. On the contrary. Combined, maybe the whole Single-Player, Co-Op, and Firefight might have been longer than Halo 3’s, but it wasn’t because the campaign was longer.
As for Halo 4 being on two discs, we won’t know until more detail is released. I wouldn’t hold my breath.
But to add to the discussion I’ll add some speculative stuff.
The only reason Halo 4 should/ would be more than one disc is if the campaign was longer. It’s most likely a different or modified engine, but I doubt they’ll do what they did for ODST. If 4 is two discs, one would be entirely the campaign and the second would be Halo 4’s Multiplayer.
But you have to take into consideration their development budget, and time. There’s only so much they can do with so much money in so much time. So they might have to cut stuff out of the campaign to save on money and time. Remember, Microsoft owns 343, so they set timelines and the budget.
But given the size limitations of the XBox discs, I’d say there’s a chance that Halo 4 could be on two discs. But, like I said, I wouldn’t hold my breath. (L.A. Noire, for example, was on 3-discs because of the limitations of the XBox disc. Whereas the PS3 Blu-Ray can fit the game on one or two discs.)