Okay so most series have a point where they stop putting semi-honest numbers in their name and start putting subtitle words that begin with RE- (Resurrection, Retribution, Redemption, Return, Reloaded, Revolutions, Recurring, Repetitive, Recon, Reach, etc.) , like the series is now an email conversation or something.
Halo, excluding the absolute worst titles but including Halo 4, has remained faithful to the number-rule of sequel nomenclature. But how long will this keep up? Almost everyone stops at 3, Call of Duty stopped at 4, Fast and Furious stopped at 1 but paradoxically went back to it for 5...
Edit: If you think they'll keep using numbers, when do you think they'll start using roman numerals?
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Probably after 6 to be honest, (unless they make spinoffs like Reach and ODST) if they still make Halo after that. GTA is weird with that as well. It went from GTA3 to GTA San Andreas, to a bunch of portable games, then GTA4.