Those two questions and more are answered in
this interview with Marcus Lehto of Bungie at CVG. It maybe another arc in the Halo storyline but in the interview with Marcus, they decided on calling it
Halo: Reach because it's a prequel to Combat Evolved and it's about the planet Reach so that's why they are going with that title instead of Halo 4.
Also, they aren't ruling out another Halo game but Bungie's next game will be a completely different IP. Now, I don't think any company ever truly walks away from an IP they created and loved so it's not surprising that he said never say never. Although, this could be Bungie's last Halo game so they are going to go all out and try to make it the best they can.
