Short answer: as of 2010 we don't know.
Bungie has commented that Reach will be their last Halo game. Halo Wars was made by a different developer, not Bungie who made the original games and was a completely different style of game altogether. However Halo 3: ODST was recently released in which you play one of the ODST Marines and Halo: Reach has recently been announced. This will be a prequel to the Halo series set at the fall of Reach, and may be considered Halo 4 as many people call Call of Duty: World at War simply COD 5, but those are nonstandard and potentially misleading names.
Halo 3: ODST is an interquel. It takes places after Halo 1 making it a "sequel" yet before Halo 3 making it a "prequel". It takes place parallel to Halo 2 after the Covenant have landed on Earth.
A true Halo 4 hasn't been announced, but considering the amount of money that can be made off of the Halo series, Halo 5 wouldn't be that much of a surprise.
To add to that, Halo authors have this weird infatuation with the number 7; no one knows why, they just do. If the hinted-at post-Reach game gets made, there will be 7 total, counting non-Bungie titles such as Halo Wars.