Nowhere near 70M unique 360 owners out there, and I'm not sure Gears of War qualifies when Microsoft says it's their most expensive game ever, as Gears isn't developed by Microsoft.
Still, though, yes, Halo 4 is going to be huge. Bigger than CoD on the 360? I doubt it, though I suppose it's possible, perhaps because of how much CoD has expanded the FPS audience. If Halo 4 does rise that much in sales over Halo 3 then I think they owe a bit of thanks to CoD. I assume you don't mean across all platforms, as that's not happening.
Halo 4's release is going to be a big deal, no question, since it doesn't have a constant, year-after-year release schedule like CoD. I think it's safe to say it's going to be the 360's biggest non-CoD title by a good margin.
