halo 4 is not a guarantee, but i would not completly say that halo 3 ended the series...i mean seriously. look at the ending for halo 3. they left the wide open possibility for a COMPLETLY different storyline. the possibility for another halo is definitly there.
There will be now Halo 4. Halo 3 was the end of the story. However, if there is a another Halo game, it will be Halo Wars(i heard a rumor somewhere). It will most definitely not be on Wii though since it still is Halo, and Microsoft owns the licsense to it.
As mentioned above Microsoft owns the rights to the game Halo. With that being said I doubt that we will ever see a Halo game made for any of the current next gen consoles. However, this doesn't mean we wont see a game produced by bungie released for the Wii. Who knows what may be in store for the Wii and or the Xbox 360 seeing as Bungie has now decided to pursue other games and possibly other consoles..