The only actors I believe that were fired were the ones who played Nikki and Paulo, and I'm fine with that, those were HORRIBLE additions to the show. Added nothing at all. The only good episode with them was when they were killed off.

"RAZZLE DAZZLE!"
I'm hearing more about Mr. Eko's leave. Apparently, the real story is that the writers Damon and Carlton said he was one of their favorite characters and they were super pissed he left and it still bothers them, that he was never really going to get killed off at all. They hired the one guy on the planet that hates Hawaii. What are the odds of that? They said a lot of the cast plans on staying there after the show is done, but from day one he wanted to get the hell back to London as soon as possible. Sounds like we won't be seeing Eko again.
Other than Eko, Nikki and Paulo, I think we can expect old cast members still making an appearance via flashback, Locke vision, ect. In fact, I think every former cast member made an appearance after they left. Even Ana Lucia will be appearing for one episode in season five.