First off, the Halo series is a trilogy as of Bungie, but Bungie has left Microsoft as an independent company so now Microsoft owns the Halo name. It's up to Microsoft to decide if they are gonna make Halo 4. But in all honesty, without Bungie, I don't think that they'd even get an 8/10 on a rating from GameSpot. Microsoft is already pushing their luck with Halo moving to a strategy field with Halo Wars.
The alternate ending on Legendary Mode does seem as though Halo 4 is a possibility but since Halo 3, I don't think they have much to go on anymore so if there is a Halo 4, it'll probably be the same as Halo 3 besides the story. Like I said though; it's up to Microsoft.