I think Mama Petrelli should stay a side character. S3 was the worst season in my opinion, and that was the only season she was a main character. In S1, S2, and S4 she was a side character and that's what she's best at because she's really not one of the Heroes. She works better as support or as a foil for Peter.
I actually think Matt can be let go. He doesn't have much reason to stay unless they shoehorn him in. Of course I'm approaching it from the angle of "whose story has been completed enough to let go" versus "who do I want to go" because I really don't want another Monica, Micah or Molly. Matt has his family, his story arc is done. Suresh is already gone basically.
Hiro and Ando could be done now considering how they both seem tired of Hero-ing and both just want to go home to Japan to be with Kimiko, but they're probably not going to get rid of Hiro or Claire. Those two and Sylar were the only characters big enough to show up on the promotional images for S4. Like it or not, the Heroes crew considers them to be the big three and they're not getting rid of them. And if Claire stays, HRG is going to stay for sure. Plus there's the fact that Claire has taken center stage in the whole "coming out" thing so for her to just disappear wouldn't make much sense.
Tracy would probably come back too. The crew is not willing to give up Ali Larter or they would have had her leave the show when Niki died.
So characters they could let go at this point: Tracy, Matt, Hiro, and Ando, Suresh.
Characters they're probably willing to let go: Suresh and maybe Matt.