Some editions of the play include a letter from O'Neill to his wife as an epigraph. It's not "officially" an epigraph, since O'Neill didn't print it as such, but he did present it to his wife as a sort of introduction to the play, and you can see why publishers think it works as an epigraph:
For Carlotta, on our 12th Wedding Anniversary
Dearest: I give you the original script of this play of old sorrow, written in tears and blood. A sadly inappropriate gift, it would seem, for a day celebrating happiness. But you will understand. I mean it as a tribute to your love and tenderness which gave me the faith in love that enabled me to face my dead at last and write this play – write it with deep pity and understanding and forgiveness for all the four haunted Tyrones.
These twelve years, Beloved One, have been a Journey into Light – into love. You know my gratitude. And my love!
July 22, 1941
The "epigraph" more than hints at the fact that we're heading into some stormy autobiographical waters. When O'Neill states to his wife that she has allowed him to "face [his] dead" at last, it makes us think there's some strong connections between O'Neill's family and "the four haunted Tyrones" (E.2).
Just checking out O'Neill's basic biography is illuminating. To begin with, his father was James O'Neill, a big-time actor. His mother was Mary Ellen Quinlan O'Neill, a sometime morphine addict. (She went by "Ella.") While O'Neill was a kid, he lived in a series of hotels with his parents, following his father around to performances. Their only semi-permanent home was a summer cottage in Connecticut. He also had two older brothers: James, Jr. (Jamie) and Edmund. Edmund died young, of measles (source).
If you've read the play, this should be ringing some major bells. You should be hard of hearing with all this bell ringing. It's almost the exact same story as Long Day's Journey. While it's still a fictional creation, this is about as autobiographical as a play can get. With all the family baggage that gets opened, it's no surprise that O'Neill wouldn't let the text be published until after his death.
The play seems to only deviate from O'Neill's life story in one major detail: the switch between his own name and that of his brother, Edmund, who died in infancy. O'Neill might've decided on this rather morbid switch for any number of reasons. Maybe he wanted to give his dead brother a chance to live. Though we'll probably never know O'Neill's true motivation for this decision, it's certainly interesting to ponder.
It's particularly poignant that O'Neill dedicated this play, with its strong themes of addiction, to his wife Carlotta. She was addicted herself to potassium bromide, a strong sedative that was available at the time. This caused unending trouble to their marriage (source). Evidently, though, it wasn't all depression and disintegration. O'Neill writes to Carlotta that their marriage has been a "Journey into Light – into love" (E.3). With the "epigraph," O'Neill lets us know that, ultimately, he wrote the play as an act of forgiveness – of the people he's loved and of himself.