While Fallout 4 could bring back characters from Fallout 3, we know for certain that one familiar face won't return. Erik Todd Dellums, the voice actor who played disc jockey Three Dog, says he won't be part of Fallout 4.
Dellums confirmed he won't be in Fallout 4 over Twitter shortly after Bethesda announced they were done recording voice work:
I let [Bethesda] know that fans were clamoring, trying to figure out if there's any chance that Three Dog would be back sometime. And, you know, they let it slip that it looks like Three Dog will. And they said that, 'Well, maybe you could tell your fans.' I don't think they even anticipated it would explode like this.
Three Dog was the disc jockey for Galaxy News Radio in the Washington D.C. area. In addition to playing music, he commented on player's good or evil decisions in quests. His presence throughout the entire game made him one of the most memorable personalities from Fallout 3. I'm sure players would have appreciated him returning for another game.
However, bringing Three Dog back wouldn't have necessarily made sense. Fallout 4 takes place in the Commonwealth, the area formerly known as Massachusetts. It would be hard for anyone to make the long trek from the Capital Wasteland to the Commonwealth, much less a noncombatant like Three Dog.
Including him in the game would also force Bethesda to make one of the Fallout 3 endings canon. If the player was good, he purified the waters of the D.C. area at the end and there would be no reason for Three Dog or anyone else to leave. That would make that region arguably the most desirable place to live in the post-apocalyptic United States. The only plausible reason he would go to the Commonwealth would be if the player had decided to poison the city's waters instead and he was forced to flee. I don't think Bethesda wants to tell players what the "right" or "wrong" ending to Fallout 3 is, though, much less make the evil ending the "right" one.
Although Three Dog won't be part of Fallout 4, players will still have at least one radio station to listen to. Leaked casting documents for the game mentioned a DJ named Travis Miles. Considering how large Fallout 4's script is, it's also possible there's more than one disc jockey servicing the Commonwealth.