So in season 4 of Game of Thrones after the fight I am unsure if the Mountain is dead or alive. And if he is alive will he be back in season 5?
The following addresses what has already happened in the TV show.
The Mountain gets poisoned in the fight, and Cersei tells Qyburn to do what he can to keep him alive. Qyburn agrees, but says he will not be the same as he was (this is consistent so far between the books and TV series). In one scene in season 5 (I don't remember the episode),
Qyburn is in his laboratory, and something underneath a sheet begins to shake. I believe this is The Mountain restrained, and possibly being revived from death/ prevented from dying by unnatural means.
This leads me to believe that he will still be in the show.
The text in the following spoiler tag is based on things that have not yet occurred in the TV show, and is based solely on what occurs in the book. However, the book and TV series have already been shown to deviate greatly, so this part of the answer might turn out to be wrong.
The Mountain is not dead, but he is also not quite alive. It doesn't elaborate greatly on what is done to him, but he is up and walking around, acting as Cersei's personal bodyguard. However, he always wears a helmet, and his face is never seen. He is unmistakeable from his size, so everyone recognizes that it is The Mountain, but it is very possible that he has become some sort of undead creature.