The whole radio idea is a nice twist to an already epic plot. Here is my check off list of people whom the voice is impossible to match. Following, I offer my theory on a few possible candidates.
The impossible list of radio voice subjects:
All of the above characters are in the same room when the last radio transmission comes through on the Walkie -Talkie.
The 'Nah List'
1. Glenn - has no motive: may come back as a hero for the final group
2. Lilly - would need a voice changer
3. Molly - would need a voice changer
4. Vernon - Would need to be insane to taunt the group; he is not crazy
5. Diana - She told the answering machine, "Take Clem to Marietta", which is northeast of Atlanta. If she were to start looking for Clem, it would be at her home then on to Marietta.
The 'could be' list
1. A bandit - Possible but a bit out of the way for scavengers
2. St. John Brother - Possible but would go after Lee directly
The 'Easter Egg List'
1. Diana, Clem's mother could be alive. Clem's father is dead from being attacked. She has no motive to harm Lee or Clem. She could not be a hostage either, as Lee has grave doubt that either of Clem's parents are still alive.
2. A family member of Lee's dead wife - This is my best guess. Here is why it makes perfect sense....
The radio guy wants Lee. Clem is collateral. This is evident by the fact that we know the radio guy already has Clem in his possession. If Clem was all the radio guy wanted, then why threaten Lee in the end? There is no money, no power, or any other thing of great value except for Lee's own life. This is what the radio guy wants... Lee. He is using Clem as bait.
From what we know of the story, Lee was recently convicted of killing. The only person(s) with a motive to kill Lee is the family member(s) of the dead victim(s). A natural course of revenge is to avenge the death(s) by killing the one person responsible... Lee.
No other character, hidden or revealed, would make sense in E4. It would insult too many people to reveal that Doug (for instance) is the radio guy.