Considering the timelines of Fallout 3 and 4 are almost during the same time period (both occurring in 2277), the only character I can imagine being in 4 would be Three Dog. Which would be awesome!

Ed-e, three dogs and fawkes. Ed-e is most useful companion ever!

That scientist lady who helped your dad restart the purifier and then dipped after we took it back from the Enclave was said to have left for the Commonwealth -

What if the dog in fallout 4 is Rex from new vegas

He will be replaced by an overpowered dog

That Rivet City guard who [spoiler]was an android from The Institute.[/spoiler]

Butch's son, after the inevitable downfall of butch's psyche following the death of his mother, riddled with Jet abuse and take-away Yao Guin pizza, he finally decided to take his uneventful life, not being able to cope with the guilt of his troubled past and the demons of his mother death that Haunted him until his finally day, one cold, radioactive night he decided to end it all, snorting all the surgery bombs he could steal that once fuelled his drug plagued lifestyle. He died a shallow, painfully slow death. Poetic justice for all the times he pushed and prodded at the lesser youths of his younger years. It was then the late, great Amata got the message, she was hollow, stuck frozen in place. Before falling to her knees after hearing the news from the overseer himself. She never told butch of their offspring, assumptions were made believing Butch given his penultimate stages wouldn't of made an adequate father. Granted, it seemed the best course of action at the time. "He's no good for no one!" She could hear the words of her mother echoing in her skull. A tear trickled down her eye as she stared at the child. A child who wouldn't know the feeling of having a father. The rush of being picked up into their fathers arm, a cold ice-cream from Mr Handy on daddy's lap, or a bedtime story of Gronac the Barbarian read by papa'. Amata fooled herself into believing she would tell butch it was his. Once he cleaned his act up. But alas, the day never came. Butch's son, led a normal, care free life, at the age of 17 he was star of the vault soccer team, and also managed a rather sturdy result of Overseer prediction on the G.O.A.T. Things were looking good for Butch's son. Until, one day. When asked by his mother, Amata to fetch mommys psycho from her jacket pocket in the wardrobe. He stumbled across something quite unsettling. From the peripheral of his vision he caught sight of a jacket, but not just any jacket, THE jacket. He pulled a face of confusion as he took it off the hanger. It was at that moment, that instance Butch's son felt an unholy rush, of euphoria and power surge through his veins. He started to breath heavy and fully take in quite what he was seeing. The jacket had some sort of gravitational pull towards him, like it was meant for him. For that one moment in time he knew what needed be of him. He knew what was expected of him, he didn't know who was ordering it, but he had a feeling, you better believe he had a feeling. Butch's son turns around, His mother Amata is stood there, face covered in tears; like the wife of a martyr after hearing the news of battle had reached the final frontier, and no one was to make it back alive. Butch's son grabbed her in his supple arms. After the tears faded away Amata told Butch's son the story of his father. Hours they sat down on the edge of Amata's bed, talking and conversing. Busts son had a mountain of questions he wanted to ask. Until this day Amata had been withdrawn about his father. Saying he left home as soon as Butch's son came into the picture. Butch's son arose from the bedside. He gave his mother a kiss on the cheek and said one finally word. Goodbye. "Where are you going, son?" "I've, I've gotta do some reflecting, I'm going out, out into the wasteland" "But you'll be killed!" His mother proclaimed. "Ive gotta do it, for him, it's what he would of wanted" "What would he of wanted?!? You didn't even know the first thing about him until now!" Butch delivered a clean right hook to his mother. Knocking her out. Tears running down his face: "I've sorry mom, I just have to, please forgive me." He gave the unconscious body of his mother a final kiss farewell. Butch's son knew what he had to do..

Bump

3dog and heard rumor of Desmond possibly in it from Point Lookout dlc

GARY!!!!

I can't remember whether she is alive or not after everything in Fallout 3, but if so then Moira. I think that was her name. As a matter of fact is Fallout 4 set before or after Fallout 3?