If it doesn't involve violence it shouldn't be traumatic because kids that age don't remember things that are not extreme. Remember, a 4 year old has no idea what sex is, or the social stigmas attached, so they would not know what is/was going on. It would just be another game to them, perhaps one they didnt like, but a game none the less. Unless someone tells them constantly later in life. What causes the trauma and consequences later in life is the behaviour of people around the victim who are aware of the crime. The best thing they could do for this kid is not mention it again, but unfortunately she watched her father beat someone to death, and that is never going away.
Sometimes things are best left in the past. My sister had a minor incident when she was around 10, it wasn't a big deal. I was there at the same time, a year older than her, and experienced the same thing, so I know exactly what happened. But later on in life as things didn't work out all that well for her personally (she was bipolar), it slowly began to take on bigger and bigger proportions, mainly (IMO) because they people around her were feading this thing as the cause of all her problems, when it in fact wasn't. They wanted to find a reason and someone to blame for her issues who wasn't my sister, and in doing so they completely missed what was really going on. Eventually she killed herself. I blame the people around her at the end. They might have meant well, but they didn't understand her true problems untill too late, but by then the damage was done.