Guys, guys, guys... settle down.
It seems as if the majority of posters in this thread haven't played the Halo games. There are easy explanations for most of the questions here.
First of all, the "5" in "Halo 5" is coming from Reach, as many have guessed - whoever made this considered Halo: Reach to take the place of Halo 4, which makes sense considering the only other games in the franchise, Halo: ODST and Halo Wars, were both spin-offs. Neither of these games were marketed or produced as a main chapter of the Halo franchise. (Halo: ODST was originally going to be Halo 3: ODST and was intended to be a $30 budget value title, placing itself more as a standalone expansion pack than as an individual title.) Thus one could consider the next main Halo game to be the fifth in the franchise. Hence Halo 5.
But we can be almost 100% certain that Microsoft will not title the next main Halo game Halo 5. First, the jump in numbering would be silly, even if you can reason out Halo: Reach taking the place of the fourth title. But more importantly, the numbered Halo games (Halo, Halo 2, Halo 3) refer specifically to Master Chief's story. That's why Halo: Reach wasn't titled Halo 4. Halo 3 was to "Finish the Fight" - specifically, Master Chief's fight.
So unless they bring back Master Chief this quickly (which would be a surprise, considering how Halo 3 ended), a numbered Halo game wouldn't make any sense. More reasonably we can expect the next new Halo game to have a subtitle a la "Call of Duty: Modern Warfare". (And besides, rumor has it that the Halo game coming out this year is some HD remake of Halo: CE.)